Make a licensing note somewhere about `pipenv check`

While writing https://github.com/pyupio/safety-db/pull/2261, I realised that commercial redistributors of pipenv should take close note of the fact that pipenv check relies on a CC-BY-NC-SA CVE database maintained by pyup.io: https://github.com/pyupio/safety-db

This means that commercial redistributors of pipenv need to choose between:

• disabling pipenv check;
• patching pipenv to use a different vulnerability database (e.g. one they maintain themselves); or
• paying pyup.io for a commercial usage license

[kennethreitz]

I’ll work on it. Unless you want to do it 😃

[kennethreitz]

Thanks for the openness @jayfk!