Add a `sampleproject/__main__.py` example

[DiddiLeija]

__main__.py is frequently used by packages and modules (like the standard turtledemo) to enable things like:

python -m some_module

I would like to propose a __main__.py example inside src/sampleproject, as a reference for new developers. I know that any change must be documented, but I don't think it would be really hard to do.

Also, if you like this idea, tell me, and I can open a pull request.

[DiddiLeija]

I know this is not user-facing, but maybe it could be a useful reference. Or maybe, we can even propose an "advanced example" to include that kind of things on it.

[di]

I would merge a PR that adds this.

[di]

There is some initial work here as well: https://github.com/pypa/sampleproject/pull/67