Add Elastic Contributor Program in Contributing Guide

Author: sethmlarson

CONTRIBUTING.rst

@@ -7,6 +7,11 @@ you would like to do. It may be that somebody is already working on it, or that
 there are particular issues that you should know about before implementing the
 change.
 
+If you want to be rewarded for your contributions, sign up for the
+`Elastic Contributor Program <https://www.elastic.co/community/contributor>`_.
+Each time you make a valid contribution, you’ll earn points that increase your
+chances of winning prizes and being recognized as a top contributor.
+
 We enjoy working with contributors to get their code accepted. There are many
 approaches to fixing a problem and it is important to find the best approach
 before writing too much code.