Add note that pySankey2 DOES NOT EQUAL the PIP version of pySankey2

[corneliusroemer]

I just lost 10min because I assumed pySankey2 can be pip installed through pip install pySankey2

This DOES NOT work, since there is already a different pip package called pySankey2.

Maybe rename this pySankey3 and put it on pip? That'd help a lot. It's not hard to put something on pip. 21 Github stars should be enough.