nit: add rate limit extension example

spec/Section 7 -- Response.md

@@ -27,6 +27,39 @@ set, must have a map as its value. This entry is reserved for implementers to
 extend the protocol however they see fit, and hence there are no additional
 restrictions on its contents.
 
+GraphQL Response for an extension with rate limit might look like this:
+
+```json example
+{
+ "data": {
+    "hero": {
+      "name": "R2-D2",
+      "heroFriends": [
+        {
+          "id": "1000",
+          "name": "Luke Skywalker"
+        },
+        {
+          "id": "1002",
+          "name": null
+        },
+        {
+          "id": "1003",
+          "name": "Leia Organa"
+        }
+      ]
+    }
+  },
+  "extensions": {
+    "rateLimit": {
+      "requestRate": 2,
+      "remaining": 98,
+      "retryAfterMs": 0,
+      "resetAfterMs": 1985
+    }
+  }
+}
+```
 To ensure future changes to the protocol do not break existing services and
 clients, the top level response map must not contain any entries other than the
 three described above.