Add FAQ about "update not wrapped in act" warnings

This has comes up every so often in issues and Spectrum, so I've tried to summarize the cause & resolutions available.

docs/react-testing-library/faq.md

@@ -178,6 +178,22 @@ snapshotDiff(firstVersion, container.cloneNode(true))
 
 </details>
 
+<details>
+
+<summary>How do I fix "an update was not wrapped in act(...)" warnings?</summary>
+
+This warning is usually caused by an async operation causing an update after
+the test has already finished. There are 2 approaches to resolve it:
+
+1. Wait for the result of the operation in your test by using [wait](https://testing-library.com/docs/dom-testing-library/api-async#wait). For example: `await wait(() => getByText("fetch completed")`.
+[an example in this issue](https://github.com/testing-library/react-testing-library/issues/441#issuecomment-520977388).
+2. Mocking out the asynchronous operation so that it doesn't trigger state updates.
+
+Generally speaking, approach 1 is preferred since it better matches the expectations
+of a user interacting with your app.
+
+</details>
+
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