misc: Updating contributing.md to discuss Gerrit guidelines

Change-Id: I2449d4b0384f33d9635fc2a314a24341816dab1d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21419
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed