Update README.md to include instructions for using test utils globally (#524)

* Update README.md

Co-authored-by: Daniel Schmidt <[email protected]>

Author: kylemh

README.md

@@ -254,7 +254,11 @@ value you can pass either a `boolean` value or a threshold between `0` and `1`.
 
 ### `test-utils.js`
 
-Import the methods from `react-intersection-observer/test-utils`.
+You can use these test utilities as imports in individual files OR you can globally mock Intersection Observer for all Jest tests. If you use a library or an application with a lot of Intersection Observer usage, you may wish to globally mock it; however, the official recommendation is to be purposeful about your mocking and do so on a per-usage basis.
+
+#### Indvidual Methods
+
+Import these from `react-intersection-observer/test-utils`.
 
 **`mockAllIsIntersecting(isIntersecting:boolean | number)`**  
 Set `isIntersecting` on all current IntersectionObserver instances.
@@ -267,6 +271,31 @@ Call the `intersectionMockInstance` method with an element, to get the (mocked)
 `IntersectionObserver` instance. You can use this to spy on the `observe` and
 `unobserve` methods.
 
+#### Global Intersection Observer Behavior
+
+##### Use Fallback
+
+You can create a [Jest setup file](https://jestjs.io/docs/configuration#setupfiles-array) that leverages the [unsupported fallback](https://github.com/thebuilder/react-intersection-observer#unsupported-fallback) option. In this case, you only mock the IntersectionObserver in test files were you actively import `react-intersection-observer/test-utils`:
+
+```js
+import { defaultFallbackInView } from 'react-intersection-observer';
+
+defaultFallbackInView(true); // or 'false' - whichever consistent behavior makes the most sense for your use case.
+```
+
+##### Mock Everywhere
+
+In your Jest config, add `'react-intersection-observer/test-utils'` to the array value for the [`setupFilesAfterEnv`](https://jestjs.io/docs/configuration#setupfilesafterenv-array) option.
+
+
+```js
+module.exports = {
+ // other config lines
+ setupFilesAfterEnv: ['react-intersection-observer/test-utils'],
+ // other config lines
+};
+```
+
 ### Test Example
 
 ```js