docs(fakeTimers): Explain how to use fake timers in testing-library

Relates to testing-library/react-testing-library#743

Author: MatanBobi

docs/using-fake-timers.md

@@ -0,0 +1,33 @@
+---
+id: using-fake-timers
+title: Using Fake Timers
+sidebar_label: Using Fake Timers
+---
+
+Using real timers in your tests is less common since they depend on real time
+lapse. For that, some testing frameworks offer the option to use fake timers in
+your tests so you won't need to depend on real times.
+
+When using fake timers in your tests, all of the code inside your test uses fake
+timers. The common pattern to setup fake timers is usually within the
+`beforeEach`, here's an example of how to do that in jest:
+
+```js
+beforeEach(() => {
+  jest.useFakeTimers()
+})
+```
+
+When doing so, you'll probably want to restore the timers after your test runs.
+For that you usually call `useRealTimers` in `afterEach`. It's important to
+remember that before calling `useRealTimers` you have to `clearAllTimers`. This
+will ensure you clear all the timers even if they weren't executed. That way,
+your fake timers are encapsulated to your tests only and when we try to cleanup,
+we will work with real timers. So you'll need to do something like this:
+
+```js
+afterEach(() => {
+  jest.clearAllTimers()
+  jest.useRealTimers()
+})
+```

website/sidebars.json

@@ -1,6 +1,6 @@
 {
   "docs": {
-    "Getting Started": ["intro", "guiding-principles"],
+    "Getting Started": ["intro", "guiding-principles", "using-fake-timers"],
     "Frameworks": [
       {
         "type": "subcategory",