Add docs

Author: leszekhanusz

docs/usage/headers.rst

@@ -6,3 +6,5 @@ If you want to add additional http headers for your connection, you can specify
 .. code-block:: python
 
     transport = AIOHTTPTransport(url='YOUR_URL', headers={'Authorization': 'token'})
+
+After the connection, the latest response headers can be found in :code:`transport.response_headers`