pep-492: Add a small example for types.coroutine

pep-0492.txt

@@ -137,7 +137,12 @@ types.coroutine()
 
 A new function ``coroutine(gen)`` is added to the ``types`` module.  It
 allows interoperability between existing *generator-based coroutines*
-in asyncio and *native coroutines* introduced by this PEP.
+in asyncio and *native coroutines* introduced by this PEP::
+
+    @types.coroutine
+    def process_data(db):
+        data = yield from read_data(db)
+        ...
 
 The function applies ``CO_ITERABLE_COROUTINE`` flag to generator-
 function's code object, making it return a *coroutine* object.