PARQUET-69: Committer doc

In order to improve turnaround time on reviewing pull requests here are a few guidelines and informations.

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

Closes #23 from julienledem/committer_doc and squashes the following commits:

d4b2e19 [Julien Le Dem] Update REVIEWERS.md
f7d8a5e [Julien Le Dem] Update COMMITTERS.md
b627d50 [Julien Le Dem] Create REVIEWERS.md
0027a98 [Julien Le Dem] Create REVIEWERS.md
6fd49d0 [Julien Le Dem] Create REVIEWERS.md
c6c26e6 [Julien Le Dem] Create REVIEWERS.md
cbbec60 [Julien Le Dem] Create REVIEWERS.md
56b7d3d [Julien Le Dem] Create REVIEWERS.md
a93ddcb [Julien Le Dem] Create REVIEWERS.md
98f1e52 [Julien Le Dem] Create REVIEWERS.md
5d960f4 [Julien Le Dem] Update COMMITTERS.md
7be5f36 [Julien Le Dem] Create REVIEWERS.md
24ef829 [Julien Le Dem] Create REVIEWERS.md
799a067 [Julien Le Dem] Create REVIEWERS.md
8ef1250 [Julien Le Dem] Create REVIEWERS.md
b85f454 [Julien Le Dem] Create REVIEWERS.md
38d1afa [Julien Le Dem] Update COMMITTERS.md
425224d [Julien Le Dem] Update COMMITTERS.md
46249ed [Julien Le Dem] Update COMMITTERS.md
7879de2 [Julien Le Dem] Create COMMITTERS.md