Add some example polyfills.

examples/with-polyfills/client/polyfills.js

@@ -1,4 +1,13 @@
+/* eslint no-extend-native: 0 */
+
 // Add your polyfills
 // This files runs at the very beginning (even before React and Next.js core)
 
 console.log('Load your polyfills')
+
+// core-js comes with Next.js. So, you can import it like below
+import includes from 'core-js/library/fn/string/virtual/includes'
+import repeat from 'core-js/library/fn/string/virtual/repeat'
+
+String.prototype.includes = includes
+String.prototype.repeat = repeat