Docs: branch support

[bpmct]

Add support for checking out a different branch (.e.g. staging) via a GIT_BRANCH option. We should explain this only applies for the first clone and users can switch branches after

[matifali]

Accepting branch explicitly would improve the UX. It can already be done when git url contains tha branch name. e.g..

https://github.com/coder/coder/#refs/heads/<branch>

Also discussed in #21

[bpmct]

Oh I see, we need to document this then