add more comments for shim and test

1. add reason for shim
2. add reason for using jest fake timers

packages/ra-input-rich-text/src/index.spec.js

@@ -44,6 +44,7 @@ describe('RichTextInput', () => {
           target: { innerHTML: '<p>test1</p>' }
         });
 
+        // ensuring the first 'text-change' event had been handled
         jest.runOnlyPendingTimers();
 
         rerender(

test-setup.js

@@ -1,5 +1,9 @@
 require('raf/polyfill');
-require('mutationobserver-shim'); // required due to quilljs
+/**
+ * As jsDom do not support mutationobserver and
+ * quill requires mutationobserver, thus a shim is needed
+ * */
+require('mutationobserver-shim');
 var enzyme = require('enzyme');
 var Adapter = require('enzyme-adapter-react-16');