Check readme suitability for PyPI

When generating sdists and wheels with the testenv:dist env in tox,
first run a check of the readme to make sure its contents will be
correctly parsed by PyPI/Warehouse. Also add dist to the set of
default envs and stop excluding the dist directory it creates from
testenv:flake8 runs since it shouldn't contain anything problematic
anyway. Clean up an unused VIRTUAL_ENV assignment in the tox.ini
too.

Update the Coder Guide with the simpler tox invocation while we're
at it.