Add contributing guidelines to pull request pages

It would be nice to have a feature like GitHub's Contributing Guidelines in Stash. We would like to use it so the team rules can be displayed when creating and reviewing pull requests. I think the convention of using a CONTRIBUTING.md file is good.

It would definitely be useful to have the whole contents of the file displayed on the create pull request page. Perhaps it could be reduced to a banner (like GitHub) on the review pull request page but with the ability to expand the banner to display the contents of the markdown file on the page when clicked.