CONTRIBUTING: note license.

This is to be more explicit about the contribution process and license.

CONTRIBUTING.md

@@ -1,5 +1,7 @@
 # Contributing
 
+Contributions to this project are [released](https://help.github.com/articles/github-terms-of-service/#6-contributions-under-repository-license) to the public under the [project's open source license](LICENSE).
+
 This project adheres to the [Open Code of Conduct][code-of-conduct]. By participating, you are expected to honor this code.
 
 [code-of-conduct]: http://todogroup.org/opencodeofconduct/#GitHub%20Markup/opensource@github.com