* Add stubs for `docopt` package
Add the single stub for the simple, but fairly well-used
[docopt](https://github.com/docopt/docopt) package.
It looks like that project is not very well maintained,
so I thought it would be easier to add the type annotations
here, as opposed to there, upstream. This has even been requested a few
times in https://github.com/docopt/docopt/issues/471. It appeared to me
that only the `docopt()` function is the public API, so I didn't include
the rest of the module's functions and attributes.
