Rewrite CONTRIBUTING.md
Rewrite CONTRIBUTING.md

The first iteration of our contribution guidelines were too heavy on
process. This is a second attempt at our development flow
in a lighter manner.

Fixes #259

saagie/mesos-dns,denen99/mesos-dns,denen99/mesos-dns,sepiroth887/mesos-dns,mcoffin/mesos-dns,vivint-smarthome/mesos-dns,mesosphere/mesos-dns,vivint-smarthome/mesos-dns,mesosphere/mesos-dns,alberts/mesos-dns,alberts/mesos-dns,sepiroth887/mesos-dns,mcoffin/mesos-dns