doc: explain GIT_REMOTE_REF in COLLABORATOR_GUIDE
Adds short mention and examples for GIT_REMOTE_REF
in COLLABORATOR_GUIDE.md
considering I couldn't easily find this info when I wanted to make sure and the COLLABORATOR_GUIDE is where I assume people would look first.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines