Update CONTRIBUTING.rst with issue commenting guidance (#897)

Added instructions for commenting on open issues before contributing.

Signed-off-by:
Justin Cappos <justincappos@gmail.com>

Author: JustinCappos

CONTRIBUTING.rst

@@ -5,6 +5,9 @@ Contributing to Repository Service for TUF WORKER
 We welcome contributions from the community and first want to thank you for
 taking the time to contribute!
 
+If you would like to work on an open issue, please first comment on the issue.  
+This helps us to ensure there are not multiple people working on the same issue.
+
 Please familiarize yourself with the `Code of Conduct`_
 before contributing.
 
@@ -37,4 +40,4 @@ Pull Request. Contributions may include:
 * Other related activities
 
 
-Check `README.rst <README.rst>`_ development section instructions.
+Check `README.rst <README.rst>`_ development section instructions.