misc: Updated CONTRIBUTING.md to discuss branches

misc: Updated CONTRIBUTING.md to discuss branches

There are some circumstances in which branches may be beneficial.
Though, in general, they should be discouraged. Therefore,
CONTRIBUTING.md has been enhanced to outline under what circumstances
creation of new branches is allowed and how they may be created and
used.

Change-Id: I2df8b38868e5c8146b068d9e7e957abbe3cf3b38
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24263
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>