Create CONTRIBUTING.md

GitHub suggests putting a CONTRIBUTING.md in the repository. https://github.com/blog/1184-contributing-guidelines

Currently, we have such content at Confluence. We can either just link people to Confluence, move those confluence pages to this file, or forget about making this file in the repo.
@moorepants it does look like you need to log in to confluence to see the coding standards. Thanks for pointing it out.  @chrisdembia, @jenhicks I believe you were considering putting the standards on github, which I think is a good idea.