Add AGPL guidelines for usage

[banesullivan]

AGPL is a complicated and widely (mis-?)interpreted license. We should add a section of the README to make sure people realize this software is under an AGPL license (much the same as the C++ TetGen source) and add a few of our interpretations/notes.

Some things to note:

• Simply importing this package into a new package might require adopting the AGPL license (e.g., import tetgen)
• Including this in your dependencies (e.g., requirements.txt or setup.py) should not require adopting the AGPL license

@jokva, if you have some time, I would greatly appreciate any guidance you may be able to provide here

[banesullivan]

Perhaps we can give some examples of usage where you do and do not need to adopt AGPL

With a big 'ol "we're not lawyers" disclaimer

[jokva]

I'm not sure if this is the place to do it, especially since it's easy to get wrong. I would say just stating it clearly in the readme (or other landing page docs) and linking to the FSF and/or other references should be fine.