Clarify in CONTRIBUTING docs that CC-BY 3.0 License applies to Bootstrap's docs

[ci skip]

CONTRIBUTING.md

@@ -197,7 +197,10 @@ included in the project:
     with a clear title and description against the `master` branch.
 
 **IMPORTANT**: By submitting a patch, you agree to allow the project owners to
-license your work under the terms of the [MIT License](LICENSE).
+license your work under the terms of the [MIT License](LICENSE) (if it
+includes code changes) and under the terms of the
+[Creative Commons Attribution 3.0 Unported License](docs/LICENSE)
+(if it includes documentation changes).
 
 ### Pull request bots
 
@@ -240,6 +243,7 @@ Run `grunt test` before committing to ensure your changes follow our coding stan
 
 ## License
 
-By contributing your code, you agree to license your contribution under the [MIT license](LICENSE).
+By contributing your code, you agree to license your contribution under the [MIT License](LICENSE).
+By contributing to the documentation, you agree to license your contribution under the [Creative Commons Attribution 3.0 Unported License](docs/LICENSE).
 
-Prior to v3.1.0, Bootstrap was released under the Apache License v2.0.
+Prior to v3.1.0, Bootstrap's code was released under the Apache License v2.0.