Basic guide to simple self-hosting

[kennsippell]

This is a very simple guide to self-hosting the DeliveryBot services. I tried to avoid the complexity of any specific hosting decisions - Docker, Kubernetes, etc. And avoid the complexity of any optional integrations like Slack etc. This seemed to be, the minimum information needed to get started self-hosting DeliveryBot.

In my experiences, the most confusing parts of the current experience was:

1. yarn build is missing from setup scripts
2. BASE_URL is needed but undocumented and missing from the .env.example
3. The basics of creating the GitHub App aren't documented. I had to string together information from several different places (github issues, community threads, manifest, probot, cb url, etc.)

cc @nomulex