Document that some installers don't install pyxl.pth

[bdarnell]

This commit doesn't work in all cases. Specifically, when installing with poetry, the destination path for all data_files is prepended with the site-packages directory, so everything gets doubled: /opt/virtualenvs/python3/lib/python3.8/site-packages/opt/virtualenvs/python3/lib/python3.8/site-packages/pyxl.pth.

There might be some way to fix this (either by detecting poetry and specifying an alternate path, or lobbying for changes to poetry to improve compatibility), but setuptools' docs say data_files is deprecated and not supported when using wheels, so unless there's another clever solution I'm not seeing it might be better to just document the issues, and the fact that you may need to import pyxl.codec.register manually (before importing any file with # coding: pyxl)

P.S. I'm the one who originally introduced the pyxl.pth trick here back when I worked at dropbox. 👋