docs: add instructions about testing with Jest

Author: CharlesMangwa

docs/FAQ.md

@@ -11,3 +11,13 @@ title: FAQ
 ## The Accelerometer always shows a positive value
 
 The Accelerometer will always show a positive z value, because this includes the gravitation from earth.
+
+## Testing with Jest
+
+The library provides a mock implementation for Jest via the following line:
+
+```js
+jest.mock('react-native-sensors', () => require('react-native-sensors/mock'))
+```
+
+If not done already, create a file named `jestSetup.js` for instance, provide its path to Jest configuration [`setupFiles`](https://jestjs.io/docs/en/configuration.html#setupfiles-array) entry and add the aforementioned line to that setup file.