Documentation custom url parameter

[Contrib1043]

Maybe add some documentation on the custom url parameter? Didn't find it in documentation, but only in #73

E.g. something like this

_ Per default overpy uses the public main api (http://overpass-api.de/api/interpreter), but you may also use a custom api server by providing the url parameter.

.. code-block:: python

import overpy

overpy.Overpass(url='http://localhost/api/interpreter')

_