Update docs - ensure readme has correct container information and mention optional dependencies

[donaldcampbelljr]

Per discussion, the docs for pipestat need some clarity:

1. Explain that the user must install the test dependencies (and optional dependencies) before running pytest.
2. Update readme to ensure it has the correct docker container info to run the db backend tests.

[donaldcampbelljr]

Ok, I updated the documentation on our new documentation site:
https://pep.databio.org/pipestat/

I did not update the pipestat documentation in this repository other than the README where I added the proper configuration information for running the pytest suite.