Document how best to manage a repository

[allanday]

Originally from flatpak/flatpak.github.io#106:

There doesn't seem to be a good document on repository best practices (some of which may require the ostree command, some of which is up to flatpak). Stuff like: versioning (tagging versions you want to keep forever, vs. ones to delete), moving versions between branches, things like that. Basically how I'd approach FlatPak repo management, from a reprepro background.

[allanday]

Also from flatpak/flatpak.github.io#106:

flatpak/flatpak#384 (comment) is also worth documenting

[TingPing]

Might be worth noting the scripts from https://github.com/ostreedev/ostree-releng-scripts

Which I use for my repo.