Add contributing docs

[apfelbox]

We need docs on how to contribute in general, how the newly introduced gulp workflow works and how you install all of it.

Docs for #269

[apfelbox]

Also, one could add a badge for Gitter to ease the communication if some questions arise.
Code:

[![Gitter chat](https://badges.gitter.im/LeaVerou/prism.png)](https://gitter.im/LeaVerou/prism)

Will look like:

[LeaVerou]

Why not create a #prismjs room on freenode and just add an irc:// link instead of gitter? Does gitter have some advantage re: discussing code?

[quantuminformation]

+1 for gitter

[anxbt]

i can generate readme.md,to show how to clone/setup the project locally.can u assign me this issue.