Added webpack tip

README.md

@@ -28,6 +28,8 @@ MobX is proudly sponsored by Mendix, Coinbase, Facebook Open Source and many [in
 * MobX >=5 runs on any browser with [ES6 proxy support](https://kangax.github.io/compat-table/es6/#test-Proxy). It will throw an error on startup on older environments such as IE11, Node.js <6 or React Native Android on old JavaScriptCore [how-to-upgrade](https://github.com/react-community/jsc-android-buildscripts#how-to-use-it-with-my-react-native-app).
 * MobX 4 runs on any ES5 browser and will be actively maintained. The MobX 4 and 5 api's are the same and semantically can achieve the same, but MobX 4 has some [limitations](#mobx-4-vs-mobx-5).
 
+_Tip: the main entry point of the MobX 5 package ships with ES5 code for backward compatibility with all build tools. But since MobX 5 runs only on modern browsers anyway, consider using `lib/mobx.es6.js`. For example by setting up a webpack alias: `resolve: { alias: { mobx: __dirname + "/node_modules/mobx/lib/mobx.es6.js" }}`_
+
 ## Translations
 
 * [中文](http://cn.mobx.js.org)