Contributing

@kenearley @stof Please share your feedback liberally.

This adds a ``contributing.md`` file which makes it clear what is expected of people opening pull requests and issues in the Chosen repository. I've used a lightly modified version of @necolas's excellent [issue-guidelines](https://github.com/necolas/issue-guidelines) because I think they're great. My sincere hope is that these guidelines will help us receive better issues and more concise pull requests to Chosen.

It is probably easier to review this document in [the browser](https://github.com/harvesthq/chosen/blob/contributing/contributing.md) than anywhere else. But do what works for you.