Add `CONTRIBUTING.md` file

Providing the `CONTRIBUTING.md` file alows us to take advantage
of GitHub's UI and prompt contributors to read the repository's
guidelines before creating an issue or opening a pull request:
https://github.com/blog/1184-contributing-guidelines

The `CONTRIBUTING.md` file content is based on @necolas's Issue
Guidelines: https://github.com/necolas/issue-guidelines
Merge remote-tracking branch 'refs/remotes/origin/master' into mr.hyde