Add FAQ about "update not wrapped in act" warnings (testing-library#255)

* 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.

* Update faq.md

* Update faq.md


Co-authored-by: Kent C. Dodds <[email protected]>

Author: grncdr

docs/react-testing-library/faq.md

@@ -126,7 +126,7 @@ If you want to make things more like shallow rendering, then you could do
 something more [like this](example-react-transition-group.md).
 
 Learn more about how Jest mocks work from my blog post:
-["But really, what is a JavaScript mock?"](https://blog.kentcdodds.com/but-really-what-is-a-javascript-mock-10d060966f7d)
+["But really, what is a JavaScript mock?"](https://kentcdodds.com/blog/but-really-what-is-a-javascript-mock)
 
 </details>
 
@@ -178,6 +178,25 @@ 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 one of
+   [the async utilities](/docs/dom-testing-library/api-async) like
+   [wait](/docs/dom-testing-library/api-async#wait) or a
+   [`find*` query](/docs/dom-testing-library/api-queries#findby). For example:
+   `const userAddress = await findByLabel(/address/i)`.
+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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