Add CONTRIBUTING.md

Motivation:
We often receive a bug report or a pull request which do not give us
enough information.  If CONTRIBUTING.md exists in the repository, Github
will display some notice in the beginning of the issue submission form,
which might increase the overall quality of the bug reports and pull
requests.

Modification:
Write CONTRIBUTING.md

Result:
Potentially higher-quality bug reports and pull requests
Update CONTRIBUTING.md