Add license requirement to _vendor/README.rst

[McSinyx]

Originally posted in GH-8330:

As pip is licensed under MIT, I believe packages shipped along (i.e. vendored) with pip must be released under MIT-compatible licenses, i.e. permissive licenses. [...] I am not a lawyer so there's a high chance that I am wrong though [...] I think our vendoring policy might want to require libraries to be vendored must be available under a MIT-compatible license. If this is agreed upon, I'll file a PR to clarify it.

I also added a lint commit to this PR, but if it's not agreed upon, I can easily drop it. Edit: the linting is moved to GH-8456. Furthermore, I'm not sure if this should be a trivial or documentation change.

[BrownTruck]

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[McSinyx]

Gentle ping!

[McSinyx]

I was thinking about this earlier today and apparently

Libraries to be vendored MUST be available under MIT-compatible licenses.

is a bit too strict, e.g. in pip's case it is OK (I think) to vendor LGPL libraries but not GPL ones.

Edit: I rephrased it, I wonder if I need to elaborate it with examples like above though.

[McSinyx]

@pradyunsg, may I have an additional review and possibly a merge?

[pradyunsg]

This is ready to merge, once the CI passes. :)

[pradyunsg]

Thanks @McSinyx! ^>^