Clean up tests, add some minimal docs to readme

[MridulS]

We shouldn't really have a copy of networkx tests in this package.

We can run the tests with:

PYTHONPATH=. \
NETWORKX_GRAPH_CONVERT=parallel \
NETWORKX_TEST_BACKEND=parallel \
NETWORKX_FALLBACK_TO_NX=True \
    pytest --pyargs networkx "$@"

[dschult]

This looks good!! :}
good to get it cleaned up too.