Add CONTRIBUTING.md to show the guidelines on Github

Github has Contributing guidelines feature: https://github.com/blog/1184-contributing-guidelines

Adding CONTRIBUTING.md helps users to choose better actions.
This is a prototype. We need to improve the words in the document.

What do you think?

edit:
I referred to AngularJS and FactoryGirl CONTRIBUTING.md
Here are some things that would increase a chance of that