Document how to use the github bot to create issues from channel

[heads-gh-bot]

This permits to refer a discussion directly from the matrix room without loosing context. Previous message permalink https://matrix.to/#/!JnRlHeXitsvPoibHwO:matrix.org/$JD59JZ107HiVP5w_npv_9A5E4cmVIIbhnOm4T56kQ64?via=dodoid.com&via=matrix.org&via=integrations.ems.host

[tlaurion]

Note that @heads-gh-bot has only read permission on the repo, meaning it can still open and close issues it created, but not close other issues he haven't created.

To interact with the bot:

!flashkeeper create <title> [description] [labels] - Create an issue for this repo
!flashkeeper assign [number] [...users] - Assign an issue to a user. If number is ommitted, the latest issue is used. If users is omitted, you are assigned.
!flashkeeper close <number> [comment] - Close an issue
!flashkeeper workflow run <name> [args] [ref] - Run a GitHub Actions workflow. Args should be specified in "key=value,key2='value 2'" format.
!flashkeeper help - This help text