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.
I think the issue is that we started the coding standards, but didn't finalize them (so they're still private to the dev team).  @aseth1 are you happy with the current listed standards, at least as a starting point that we can build out later?