feat: Simpler integration with the new setupFilesAfterEnv jest feature (#87)

* feat: Simpler integration with the new setupFilesAfterEnv jest feature

* docs: Talk about using the extend-expect import

Author: maraisr

README.md

@@ -54,6 +54,12 @@ test('snapshot difference between 2 React components state', () => {
 });
 ```
 
+... alternatively import it once, for instance in your [tests setup file](https://jestjs.io/docs/en/configuration.html#setupfilesafterenv-array):
+
+```js
+require('snapshot-diff/extend-expect');
+```
+
 Produced snapshot:
 
 ```diff

extend-expect.js

@@ -0,0 +1,3 @@
+const { toMatchDiffSnapshot } = require('./build/');
+
+expect.extend({ toMatchDiffSnapshot });