Explain the Any type in more detail

[JukkaL]

The PEP draft talks about Any as a union type, but in mypy it's actually very different from a union type. For example:

• List[Any] is compatible with List[int] (and vice versa), but this is not true for union types.
• A value of type Any can be assigned to a variable with an arbitrary type and vice versa, which is not true for union types.

I think we should be more explicit about at least some of the expected properties of the Any type.

[gvanrossum]

Duplicate of #38 :-) Added to README.rst. Closing.