website: Updated contributed guidelines with `stable` branch.

Change-Id: Ib708ba775862f15645adb9cb3b4155a9bf5951f2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/35435
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
1 file changed