Add a section mentioning subscriptions to the docs

Author: Cito

docs/usage/other.rst

@@ -1,3 +1,17 @@
+Subscriptions
+-------------
+
+Sometimes you need to not only query data from a server, but you also want
+to push data from the server to the client. GraphQL-core-next has you also
+covered here, because it implements the "Subscribe" algorithm described in
+the GraphQL spec. To execute a GraphQL subscription, you must use the
+:func:`graphql.subscribe` method from the :mod:`graphql.subscription` module.
+Instead of a single ``ExecutionResult``, this function returns an asynchronous
+iterator yielding a stream of those, unless there was an immediate error.
+Of course you will then also need to maintain a persistent channel to the
+client (often realized via WebSockets) to push these results back.
+
+
 Other Usages
 ------------