adding Contrib.md

First attempt, based on https://github.com/mathjax/MathJax/wiki/draft-for-CONTRIBUTING.md#conduct
I'm not sure I like the rebase suggestion here.  I'd rather see several small commits that are easy to understand than one larger one that shows the whole thing.  The pull request on github will show the full changes anyway, so I'm not sure what this accomplishes.  The pull request comment can be used to do what the single commit message would have done.  So I'd be tempted to drop this link.