Add contribution guidelines

CONTRIBUTING.md

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+# CONTRIBUTING
+
+## Introduction
+
+This document explains how to contribute to Invidious.
+
+Each section below describes a different way to contribute, based on your skills and experience
+with Invidious. Here is a summary:
+
+* [Issues](#issues): You are a regular user, and want to report a bug, ask for a new feature or for
+  an existing feature to be improved.
+
+* [Translations](#translations): You speak English and you're fluent in another language, and want
+  to help us reach more users around the globe.
+
+* [Hosting an instance](#hosting-an-instance): You're a sysadmin and have some time and server
+  ressources to spare, with some prior experience with invidious.
+
+* [Testing](#testing): You're an advanced user, who is already running their own instance, is not
+  worried of compiling Invidious from source, and wants to help us test bug fixes or new features.
+
+* [Development](#development): You're a developper and want to contribute code, or help with
+  reviewing the code written by others.
+
+
+
+## Issues
+
+We use the [Github issue tracker](https://github.com/iv-org/invidious/issues) to track and manage
+bug reports, feature requests and improvements.
+
+**Note: Before opening any kind of issue, make sure to search on the tracker
+with various keywords to verify that no other similar issue have already been
+opened (and/or closed).**
+
+In order for everyone to be able to understand eachother, all exchanges are done in English.
+You can obviously use a translator if needed.
+
+Please be polite and respectful in your exchanges. Remember that we're volunteers providing that
+service for free. We don't have the time nor energy to deal with bad manners.
+
+
+### Bug reports
+
+The most common case is that you ended up on a page saying "you encountered a
+bug in Invidious" while you were browsing a channel or watching a video.
+
+In that case, you should open a "bug report". Please include as many details as possible, so
+that we can easily reproduce your problem on our side.
+
+Before opening your issue, **make sure to test a few other instances**, just to check that the
+problem you're facing is not temporary (E.g an overloaded instance, a network outage, etc.) or
+caused by configuration error on your side.
+
+If a bug report already exists for your problem, you can add a comment with more details, but make
+sure that it's useful and adds value to the discussion. If 20 people did that already, it might not
+be relevant to post a new comment.
+
+Here is a non-exhaustive list of details that will help us:
+
+  * **A clear and concise list of steps to reproduce the problem**
+  * A link to the page where the bug happened
+  * Browser/OS version, device type (mobile, desktop, etc..)
+  * Were you logged in?
+  * If the bug is caused by an external file (ex: when importing subscriptions), try to include
+    that too (Get in touch with us if you want to share these files privately).
+  * If you're hosting your own instance, include relevant config file(s).\
+    **Make sure to redact secrets like your DB password or HMAC key first!!**
+
+
+**Note: Security-related issues should be reported by e-mail at
+[security@invidious.io](mailto:security@invidious.io).**
+
+
+### Enhancement
+
+If you feel that some existing feature can be improved, you should open an "enhancement" issue.
+
+In your issue, describe what the Invidious currently does, what you don't like about it, and propose
+ways to improve that feature. If you can provide screenshots or drawings to support your explanation
+that's even better!
+
+Please be aware that Invidious is heavily aimed towards simplicity and being usable without
+Javascript. It means that we have to deal with compromises all over the place, and some features
+might not be as good as JavaScript-rich alternatives (like Freetube), that allows much more
+flexibility.
+
+
+### Feature resquests
+
+If you think that Invidious lacks some feature or another, you should open a "feature request".
+
+Do note that Invidious is and will remain a Youtube-only front-end. Requests to add support for
+other services (e.g: Bandcamp, [Odyssee](https://github.com/iv-org/invidious/issues/3022)) will be
+systematically rejected.
+
+
+
+## Translations
+
+Invidious is translated in many languages using Weblate:
+https://hosted.weblate.org/projects/invidious/.
+
+We recommend creating an account or connecting with one of the other authentication providers that
+Weblate supports (Github, GitLab, Google, etc..) for a better experience.
+
+We also accept translation updates using Pull requests, but please be aware that it represents more
+work for us to merge those.
+
+
+
+## Hosting an instance
+
+Another way to contribute to invidious is to host a [public instance]
+(https://instances.invidious.io/).
+
+To do so, you need a server (either a VPS or dedicated) with
+[enough ressources](https://docs.invidious.io/installation/#hardware-requirements) to handle the
+load. You'll also need a domain name with a valid TLS certificate (e.g provided by Let's Encrypt).
+
+Then, if your instance follows the
+[rules listed here](https://docs.invidious.io/instances/#rules-to-have-your-instance-in-this-list),
+you can request your instance to be added to the list by
+[creating an issue on the documentation repository](https://github.com/iv-org/documentation/issues).
+
+Once you've filled the form with your instance's informations, your instance will be added to our
+uptime monitor. From there, a probatory period of 30 days will start, to make sure that you can
+keep your instance online and up to date. Finally, your instance will be added to the list. We'll
+ask you to join our Matrix room, so that we can inform you of important updates and exchange with
+other maintainers.
+
+
+
+## Testing
+
+Once reviewed, the new features or bug fixes must be tested before being merged. In general, this
+can be achieved by running the following commands (change the PR number as required):
+```bash
+git clone https://github.com/iv-org/invidious
+cd invidious
+wget "https://github.com/iv-org/invidious/pull/4439.diff"
+git apply 4439.diff
+docker compose up
+```
+
+If you have prior knowledge of Invidious, that's great, but otherwise feel free to get in touch
+with us on Matrix or IRC. We'll do our best to give you a better understanding of the project.
+
+Once you have deployed the patch on your test instance, try the changes mentionned in the pull
+request. Often times, the linked issue might contain examples of channels/comments/videos impacted.
+Definitely use those to check that the behaviot you see is the expected one. After that, add a
+comment on that PR with your test methodology and your findings. Add screenshots (for interface
+changes) and code snippets (for API changes) as needed.
+
+
+
+## Development
+
+Code contributions are handled through
+[Github's Pull Requests (PRs)](https://github.com/iv-org/invidious/pulls).
+
+Invidious' backend is developped in Crystal, a compiled language inspired from Ruby. The frontend
+is developped using Crystal's own templating engine (ECR) with some vanilla JS and CSS.
+
+Invidious follows more or less closely the [Crystal coding convention]
+(https://crystal-lang.org/reference/latest/conventions/coding_style.html#directory-and-file-names),
+except for the "Directory and File Names" sections.
+
+
+### Contributing code
+
+TODO
+
+
+### Reviewing code
+
+TODO