Add CONTRIBUTING.md with workflow

https://github.com/blog/1184-contributing-guidelines

I just copied the content of the workflow document for now and changed the unaviable links (http ://github,com/github/git-reference/issues/71).

While links to the corresponding pages are in the readme, they might be overlooked, if one does not want to drive into core development and just opens a pull-request or an issue. I'm not sure that the document in it's current form is ideal as it would appear on pull-requests *and* issues. Having some short description as [symfony](https://github.com/symfony/symfony/blob/master/CONTRIBUTING.md) or the structure [zf2](https://github.com/zendframework/zf2/blob/master/CONTRIBUTING.md) uses might be better suited.

However, if directions should be more exact. We could create some wiki-articles for different scenarios (writing documentation using GitHub for Windows, the WebInterface, PhpStorm, MarkPad, or doing small fixes etc.).

Tbh.: I dislike gifs, but I created this 7 months ago, if this could be usefull I'd rework and extend it to a series of wiki entries.
https://github.com/suralc/yii2/wiki/Contribute:-Small-and-Doc-fixes