Simplify CONTRIBUTING.md 

In an effort to keep all of the documentation surrounding contributing to YUI in the GitHub wiki, and to simplify the process of updating the contributing documentation, I was asked to update the CONTRIBUTING.md. It now points to the improved [Contributing to YUI document ](https://github.com/yui/yui3/wiki/Contributing-to-YUI ) as well as to the updated [Contribution Standards](https://github.com/yui/yui3/wiki/Contribution-Standards) for pull request standards.

This helps simplify the update process for this document and removes the merge/pull overhead. It also clarifies the description of the project and references the Gallery.