Adding CONTRIBUTING.md Fixes #857

Moving the content finally from the wiki into the repo under CONTRIBUTING.md
I know this is "documentation"  but since it's a new file doing this as a pull request to do due diligence.
This is verbatim from the wiki file that I've been soliciting and incorporating feedback on for several months from the yui-contrib mailing list. ( https://github.com/yui/yui3/wiki/Contributing.md )