PROTON-316: Provide developer instructions.

The DEVELOPERS file provides instructions for developers who want to use
the Proton code within a development environment. It includes
instructions for setting up their development environment (including
setting the CPROTON_BUILD environment variable), how to submit patches
and also for subscribing to the Proton mailing list.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1485315 13f79535-47bb-0310-9956-ffa450edef68