Document how to silence flake8 etc. using # noqa

This is a common issue for people using type comments:

```py
from typing import List  # Linter complains about unused import
a = []  # type: List[int]
```

`# noqa` helps with flake8 (and maybe other linters):

```py
from typing import List  # noqa
a = []  # type: List[int]
```

Document this is common issues.