misc: CONTRIBUTING.md to advise linking Jira Issues in commits

misc: CONTRIBUTING.md to advise linking Jira Issues in commits

Jira Issue: https://gem5.atlassian.net/browse/GEM5-186

Change-Id: I9fa1471627aed139c0ff207e35a32d9faa82eeb8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23261
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>