Document Empress' existing python API

It's currently possible to use Empress through Python, even outside of QIIME 2 (so long as all needed dependencies are installed). This is already done in the make-dev-page.py test script. However, this isn't really advertised anywhere.

It'd be nice to add a Jupyter Notebook (similar to what Emperor has in its repository) that documents this API. Linking such a notebook from the README would, in tandem with #425, probably help expand the tool's user base a decent amount.