Add A CONTRIBUTING.md File

Hi. I noticed that a lot of other repositories have CONTRIBUTING files to give guidelines to people making issues or pull requests. I've made a skeleton with what I think people should do at bare before/when making an issue. I'm less certain what we'd like to see from PRs, thoughts on that?

You can see more on this at: https://github.com/blog/1184-contributing-guidelines .

An example of a very detailed one is YCMs at: https://github.com/Valloric/YouCompleteMe/blob/master/CONTRIBUTING.md . I don't think we want anywhere near that much text, but it's an example.