Add some semi-legal gibberish to CONTRIBUTING.md

CONTRIBUTING.md

@@ -63,6 +63,17 @@ should be asked on the
 - Submit a pull request
 ([how to create a pull request](https://help.github.com/articles/fork-a-repo))
 
+By contributing code to CodeMirror you
+
+ - agree to license the contributed code under CodeMirror's [MIT
+   license](http://codemirror.net/LICENSE).
+
+ - confirm that you have the right to contribute and license the code
+   in question. (Either you hold all rights on the code, or the rights
+   holder has explicitly granted the right to use it like this,
+   through a compatible open source license or through a direct
+   agreement with you.)
+
 ### Coding standards
 
 - 2 spaces per indentation level, no tabs.