New contributions document

Hi everyone,

Thank you so much for your interest in the Core Libraries over the first 2 days! Your response to the release has been overwhelming, and we’re really looking forward to working with you on this project going forward.

I quickly realized that we needed to clarify some of the process that we want to use for accepting contributions, so I put a new file in swift-corelibs-foundation to outline it. The summary is that we follow the Swift process, outlined at here: - Contributing

We have a few additional things to think about, which I’ve written up in the document. Specifically, I want to make sure we follow the swift-evolution process for API changes in the project. I’m going to go through some of our pull requests that propose change to public API and note that we should discuss it here and on the swift-evolution list before proceeding with a code pull.

Thanks again,
- Tony