Add a note about the typing module and gradual typing

[amontalenti]

I was listening to a Python coding standards talk today and there was a lot of discussion of typing in 3.x, mypy, and other related tools. This obviously introduces a slew of style questions -- off the top of my head:

• should you introduce typing to a project, even if it mixes typed and non-typed code?
• should you use mypy or similar type checkers as a standard part of a Python linting process, akin to using flake8?
• how should the documentation advice (with reST and Sphinx etc.) be revised now that type annotations can be used directly on the code?

[amontalenti]

#48 is related.