Provide CONTRIBUTING documention

Summary:
Documents how to contribute bug reports, feature requests, new/modified
code and how code reviews are conducted on HepForge Phabricator

Documentation for admins for creating a new release are also provided.

Closes T123

Test Plan: Ask Alexei to follow these docs when submitting his code changes for review

Reviewers: jback, kreps, abudinen

Reviewed By: jback

Tags: #evtgen

Maniphest Tasks: T123

Differential Revision: https://phab.hepforge.org/D86