Document how to silence flake8 etc. using # noqa

[JukkaL]

This is a common issue for people using type comments:

from typing import List  # Linter complains about unused import
a = []  # type: List[int]

# noqa helps with flake8 (and maybe other linters):

from typing import List  # noqa
a = []  # type: List[int]

Document this is common issues.

[JukkaL]

This came up in #4302 most recently but has been a common issue at Dropbox as well. At Dropbox we have custom lint hacks to make linters not complain about imports only used within type comments, but they aren't publicly available, as far as I know.

[tjerkw]

@JukkaL how did you fix it at dropbox? Is there something out there we can use? I dislike the #noqa comments in the code.

[JukkaL]

@tjerkw I didn't work on that fix, but I have a vague recollection about adding a linter pass that looks for type comments and adds dummy references to every name used in a type comment to the end of the source file before actual linting. @gvanrossum may know more.