Make the global installation clearer (#323)

Add the other necessary part of the setup for global use.

Author: RobinKnipe

README.md

@@ -471,12 +471,18 @@ import 'jest-styled-components/native'
 
 # Global installation
 
-It is possible to setup this package for all the tests. Import the library once in the `src/setupTests.js` as follows:
+It is possible to setup this package for all the tests. For Example: import the library once in the `src/setupTests.js` as follows:
 
 ```js
 import 'jest-styled-components'
 ```
 
+...then add the following to `test.config.js`:
+
+```js
+  setupFilesAfterEnv: ['<rootDir>/src/setupTests.js']
+```
+
 # Working with multiple packages
 
 If Jest Styled Components is not working, it is likely caused by loading multiple instances of `styled-components`. This can happen especially when working with a Lerna monorepo. Starting with `[email protected]`, a warning will be logged when multiple instances of it are being included and run as part of the Jest tests. Using `[email protected]` and lower with multiple instances will cause a silent error with unexpected results.