doc: collaboration: code: gerrit commit message JIRA key

Added line for JIRA key in the commit message and a note about
the JIRA key.

Change-Id: I8b447d42a592a1ba88a1cc476fbb563365a4316f
Signed-off-by: Javier B Perez <javier.b.perez.hernandez@intel.com>
1 file changed