PARQUET-6: Create documentation on how to contribute.

Author: Julien Le Dem <julien@twitter.com>

Closes #8 from julienledem/master and squashes the following commits:

aa0d751 [Julien Le Dem] correct mailling-list
285b39c [Julien Le Dem] Update CONTRIBUTING.md
bca9bc4 [Julien Le Dem] Create CONTRIBUTING.md