Add docs for running molecule tests locally
When the ansible-pulp3 molecule tests fail on travis, the error message is rarely helpful.
The way that I test list locally is to pip install molecule into a virtualenvironment (on my machine, not the vm).
`molecule test --all` runs all scenarios (right now, this is "default" and "source")
`molecule test --scenario-name source` only tests the source installation
`molecule test` actually runs a collection of tasks, (create, prepare, converge, idempotence, test, delete, etc)
if our playbooks fail to run correctly, `molecule converge` only runs a subset (create, prepare, converge) so the tests don't need to run so long.
Any of these can include `--debug`, as in `molecule --debug test` to retrieve a more helpful error message. However, it is best to do this on only a subset because --debug is extremely verbose.