add packaging guide for aio app pack (#320)

Co-authored-by: Jesse MacFadyen <purplecabbage@gmail.com>

gatsby-config.js

@@ -98,6 +98,10 @@ module.exports = {
           {
             "title": "Event hooks for App Builder Applications",
             "path": "guides/app-hooks.md"
+          },
+          {
+            "title": "Packaging for Developer Distribution",
+            "path": "guides/packaging.md"
           }
         ]
       },

src/pages/guides/packaging.md

@@ -0,0 +1,46 @@
+---
+keywords:
+  - Adobe I/O
+  - Extensibility
+  - ISV
+  - Developer Tooling
+---
+
+
+# Packaging for Developer Distribution
+
+To get your App Builder app listed for [Adobe Developer Distribution](https://developer.adobe.com/developer-distribution/), you will need to package it.
+
+The `aio app pack` command will verify and bundle your app. In the root of your app folder, run this command:
+
+```sh
+aio app pack
+```
+
+After this command completes running, you can find the app package in your app folder as: `dist/app.zip`.
+
+## App Validation
+
+1. `app.config.yaml` 
+     - will be checked if it is in a valid format, and will show specific config errors for you to fix, if necessary
+2. `package.json` version
+     - application version format must be `X.Y.Z`, where X, Y, and Z are non-negative integers
+3. files to be packaged. All the files in your app folder will be packaged EXCEPT:
+     - files specified in `.gitignore`
+     - files specified in `.npmignore`
+     - any `dist` folders
+     - any dot files (.env, .gitignore, etc)
+     - any OS junk files (.DS_Store, thumbs.db, etc)
+4. event registrations will be validated (if any)
+
+## Hooks
+
+Two new [hooks](./app-hooks.md) have been added for this command, and will run before and after the command is run (respectively):
+
+1. pre-pack
+2. post-pack
+
+Your hook handler function will be passed two items:
+
+1. `appConfig` (object) - this contains the config of the current application
+2. `artifactsFolder` (string) - this will be the location of the folder containing all the packaging artifacts that will be bundled