Clean single-source support for Python 3 and 2 - doc

 Future is the missing compatibility layer between Python 2 and Python 3. It
 allows one to use a single, clean Python 3.x-compatible codebase to support
 both Python 2 and Python 3 with minimal overhead.
 .
 The imports have no effect on Python 3. On Python 2, they shadow the
 corresponding builtins, which normally have different semantics on Python 3
 versus 2, to provide their Python 3 semantics.
 .
 This package contains the documentation.