Added more concrete information on how to contribute.
Added more concrete information on how to contribute.

- Added step by step instructions on how to contribute, either by
clon ing or by patch.

- Added section on contributing to the documentation and how to
improve it.

- Made minor changes on the code style and English.

- Enphatised the difference between the mailing list and announcement
lists.

theblacklion/pyglet,oktayacikalin/pyglet,theblacklion/pyglet,theblacklion/pyglet,theblacklion/pyglet,oktayacikalin/pyglet,oktayacikalin/pyglet,oktayacikalin/pyglet,theblacklion/pyglet,oktayacikalin/pyglet