Add project description

[a-recknagel]

The readme (and as an extension of it, the docs) doesn't have a project description yet. It would be nice to have, in order to know what is actually the goal, and how the project may help people.

Draft: Have something like this in it

Before:

from typing import List, Tuple, Union, Optional

def foo(var: Tuple[Union[bool, str], Optional[bool]]) -> List[Union[bool, str, None]]:
    ...

After:

import stenotype

def foo(var: (bool|str, bool|None)) -> [bool|str|None]:
    ...