add subscribe example

stream/examples.json

@@ -9,5 +9,17 @@
             "message": {"id": "1", "type": "signup", "user": "john"}
         },
         "response": {}
+    }],
+    "subscribe": [{
+        "title": "Subscribe to a topic",
+        "description": "Subscribe to messages on a given topic from the stream",
+        "run_check": false,
+        "request": {
+            "topic": "events"
+        },
+        "response": {
+            "topic": "events",
+            "message": {"id": "1", "type": "signup", "user": "john"}
+        }
     }]
 }