Add `CONTRIBUTING.md`

Provide a `CONTRIBUTING.md` file in order 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 content in the `CONTRIBUTING.md` file is based on @necolas's
Issue Guidelines: https://github.com/necolas/issue-guidelines.
 Add Fake logo