Add CONTRIBUTING.md

GitHub will display a link to this file over the text box for creating a
pull request or an issue.
CONTRIBUTING.md: add commit message guidelines
CONTRIBUTING.md: add “don’t worry about it”
CONTRIBUTING.md: add “Commits” section

and make “Commit messages” a subsection
CONTRIBUTING.md: Use some non-breaking spaces
CONTRIBUTING.md: Move “don’t worry” to top

I’m about to add some more rules / guidelines, and I’m afraid of scaring
off people.
CONTRIBUTING.md: Please commit tests separately
Merge pull request #4 from ceylon/contributing.md

Add CONTRIBUTING.md