Add CONTRIBUTING.md

In #121, it was brought up that having a `CONTRIBUTING.md` would help guide people to the right places when wanting to get involved with the PHP-FIG, in accordance with the [Github Contributing Guidelines](https://github.com/blog/1184-contributing-guidelines).

The proposed file contains just one key paragraph from the [FAQ](http://www.php-fig.org/faq/), along with a link to it in order to keep duplicate documentation down.