Add skeleton CONTRIBUTING.md

Refs #3

Author: cvrebert

CONTRIBUTING.md

@@ -0,0 +1,4 @@
+# Contributing
+
+## Licensing
+By contributing to this project/repository, you agree to dual-license your contribution under [the MIT License](https://github.com/browser-bugs/browser-bugs.github.io/blob/master/LICENSE-MIT.txt) and the [Creative Commons Attribution 4.0 International Public License](https://github.com/browser-bugs/browser-bugs.github.io/blob/master/LICENSE-CC-BY.txt).