Doc, add travis badge to readme.md
Add CONTRIBUTING.md

As per github.com's advice, a `CONTRIBUTING` is a common place to find
guidelines on how to contribute to an open source project.

Add a `CONTRIBUTING.md` with instructions for:
* Submitting a contributor license agreement
* How to setup locally for development
* Etiquette for commits on this project

https://github.com/blog/1184-contributing-guidelines