Add docs for route params convertors

docs/source/quickstart.rst

@@ -48,6 +48,14 @@ If you want dynamic URLs, you can use Python's familiar *f-string syntax* to dec
 
 A ``GET`` request to ``/hello/brettcannon`` will result in a response of ``hello, brettcannon!``.
 
+Type convertors are also available::
+
+    @api.route("/add/{a:int}/{b:int}")
+    async def add(req, resp, *, a, b):
+        resp.text = f"{a} + {b} = {a + b}"
+
+Supported types: ``str``, ``int`` and ``float``.
+
 Returning JSON / YAML
 ---------------------