Thorough overhaul of CONTRIBUTING doc.

The doc has been reorganized and reordered. New sections have
been added as necessary to cover things not covered by the old
guide. Some sections were expanded (e.g. detailing differences
between E2E and System tests). Some sections that we did not
actually follow were removed.

Fixes https://issues.redhat.com/browse/RUN-2281

Signed-off-by: Matt Heon <mheon@redhat.com>