Mention functools' cached_property in the readme

[pydanny]

This will help remove ambiguity over what to use.

[martin-thoma]

I've seen several cached_property sources:

• functools.cached_property (core python)
• Django: django.utils.functional
• Flask (werzeug)
• PyPI cached_property (this project)

I'd be pretty happy about an overview :-)

[mgorny]

Yes, please. functools.cached_property seems to be actually equivalent to threaded_cached_property, so a comparison would be nice.

[pydanny]

I'm open to a PR that makes the necessary changes to the README.