doc for system hook merge_requests_events api

doc/api/system_hooks.md

@@ -55,6 +55,7 @@ POST /hooks
 | `token` | string | no | Secret token to validate received payloads; this will not be returned in the response |
 | `push_events` | boolean |  no | When true, the hook will fire on push events |
 | `tag_push_events` | boolean | no | When true, the hook will fire on new tags being pushed |
+| `merge_requests_events` | boolean | no | Trigger hook on merge requests events |
 | `enable_ssl_verification` | boolean | no | Do SSL verification when triggering the hook |
 
 Example request:
@@ -73,6 +74,7 @@ Example response:
     "created_at":"2016-10-31T12:32:15.192Z",
     "push_events":true,
     "tag_push_events":false,
+    "merge_requests_events": true,
     "enable_ssl_verification":true
   }
 ]