Merge pull request #1682 from github/contributing-license

CONTRIBUTING: note license.

CONTRIBUTING.md

 Contributing to hub
 ===================
 
+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 uphold this code.
 
 [code-of-conduct]: http://todogroup.org/opencodeofconduct/#Hub/opensource@github.com