Documentation on the code & project management process

Describes a workflow for tickets - issues and pull requests - on
GitHub. This workflow uses a system of labels that help provide
key at-a-glance information about the state of any ticket.

In particular, the system is intended to provide unambiguous
information about a ticket's:

* status: a ticket's health and what stage the ticket is at in
the workflow
* needs: what next actions are required to move it forward

The existing labels on GitHub have been renamed and regrouped to
conform to this system.
Merge pull request #3657 from evildmp/management-docs

Documentation on the code & project management process