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>

CONTRIBUTING.md

@@ -1,6 +1,7 @@
 Authors: Jason Lowe-Power
          Andreas Sandberg
          Steve Reinhardt
+         Bobby R. Bruce
 
 If you've made changes to gem5 that might benefit others, we strongly encourage
 you to contribute those changes to the public gem5 repository. There are
@@ -179,6 +180,11 @@ Signed-off-by tag to the commit message. By adding this line, the contributor
 certifies the contribution is made under the terms of the Developer Certificate
 of Origin (DCO) [https://developercertificate.org/].
 
+If your change relates to a [Jira Issue](https://gem5.atlassian.net), it is
+advised that you provide a link to the issue in the commit message (or messages
+if the Jira Issue relates to multiple commits). Though optional, doing this
+can help reviewers understand the context of a change.
+
 It is imperative that you use your real name and your real email address in
 both tags and in the author field of the changeset.