Adding contributing.rst

[cafferata]

Adding an contributing step by step guide, this will fix #29.

[cmodijk]

Maybe change the git clone part to first fork and then clone the fork to make sure they can create a pull request? Like the php-http documentation?

• http://docs.php-http.org/en/latest/development/contributing.html#sending-a-pull-request

[cafferata]

Added the git fork part based on the php-http documentation example.

[Nyholm]

👍
This is "contribute to the docs". We should also have a "contribute to Simplebus"