Update docs to mention naming conflicts with imported python modules

[greenaar]

Describe the bug A clear and concise description of what the bug is. shows using lib/ for modules. This is fine UNLESS you are using the slack backend, which has a lib.py which occurs earlier in the search path.

The resulting error looks like an import failure, ModuleNotFoundError: No module named - when actually it's a naming collision.

To Reproduce See https://github.com/errbotio/errbot/issues/1687 for the discovery process

Expected behavior The docs should choose a new name, OR highlight that the lib/ name could already be taken.

If desired, I'm happy to attempt an update to the docs with some verbiage around potential naming collisions.