Add architecture.md

There are a couple of things to explain how the:

- [ ] django, bot and worker are set up
- [ ] how to run it locally
- [ ] how the deployment works
- [ ] how the github integration is set up (because it involves extra API calls)
- [ ] how and where to add tests
- [ ] how to add a bot command
- [ ] diagrams of all the components
- [ ] diagram of the deployment