Note how to access Perf in docs

Author: glenjamin

docs/docs/09.8-perf.md

@@ -16,6 +16,8 @@ In addition to giving you an overview of your app's overall performance, ReactPe
 
 ## General API
 
+The `Perf` object documented here is exposed as `React.addons.Perf` when using the `react-with-addons.js` build in development mode.
+
 ### `Perf.start()` and `Perf.stop()`
 Start/stop the measurement. The React operations in-between are recorded for analyses below. Operations that took an insignificant amount of time are ignored.