Document test renderer act (temporary fix)

content/docs/addons-test-utils.md

@@ -46,6 +46,10 @@ var ReactTestUtils = require('react-dom/test-utils'); // ES5 with npm
 
 To prepare a component for assertions, wrap the code rendering it and performing updates inside an `act()` call. This makes your test run closer to how React works in the browser.
 
+>Note
+>
+>If you use `react-test-renderer`, it also provides an `act` export that behaves the same way.
+
 For example, let's say we have this `Counter` component:
 
 ```js
@@ -342,4 +346,4 @@ ReactTestUtils.Simulate.keyDown(node, {key: "Enter", keyCode: 13, which: 13});
 >
 > You will have to provide any event property that you're using in your component (e.g. keyCode, which, etc...) as React is not creating any of these for you.
 
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