Commit message format in contributing guidelines

Commit message format in contributing guidelines

Add the need to have a commit message with a specific
format to the contributing guidelines. Provide a script
to help enforce commit message style.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5943