[RFC#236] Add guide for String prototype extensions deprecation

content/ember/v3/ember-string-prototype-extensions.md

@@ -0,0 +1,63 @@
+---
+id: ember-string.prototype-extensions
+title: Deprecate String prototype extensions
+until: '4.0.0'
+since: 'Upcoming Features'
+---
+
+Calling one of the [Ember `String` methods](https://api.emberjs.com/ember/3.22/classes/String) (camelize, capitalize, classify, dasherize, decamelize, underscore) directly on a string is deprecated.
+
+While Ember addons (`ember addon …`) have prototype extensions disabled by default, they are enabled for applications (`ember new …`) making you able to call `"Tomster".dasherize()`, for example.
+Instead of calling the method on the string, you should instead import the function from `@ember/string`.
+
+Before:
+
+```js
+let mascot = "Empress Zoey";
+
+mascot.camelize();   //=> "empressZoey"
+mascot.capitalize(); //=> "Empress Zoey"
+mascot.classify();   //=> "EmpressZoey"
+mascot.decamelize(); //=> "empress zoey"
+mascot.underscore(); //=> "empress_zoey"
+mascot.w();          //=> [ "Empress", "Zoey" ]
+```
+
+After:
+
+```js
+import  {
+  camelize,
+  capitalize, 
+  classify, 
+  decamelize,
+  underscore, 
+  w, 
+} from "@ember/string";
+
+let mascot = "Empress Zoey";
+
+camelize(mascot);   //=> "empressZoey"
+capitalize(mascot); //=> "Empress Zoey"
+classify(mascot);   //=> "EmpressZoey"
+decamelize(mascot); //=> "empress zoey"
+underscore(mascot); //=> "empress_zoey"
+w(mascot);          //=> [ "Empress", "Zoey" ]
+```
+
+You may also instead rely on methods from another library like [lodash](https://lodash.com/).
+Keep in mind that different libraries will behave in slightly different ways, so make sure any critical `String` transformations are thoroughly tested.
+
+You can also [disable String prototype extensions](https://guides.emberjs.com/release/configuring-ember/disabling-prototype-extensions/) by editing your environment file:
+
+```js
+// config/environment.js
+ENV = {
+  EmberENV: {
+    EXTEND_PROTOTYPES: {
+      Date: false,
+      String: false,
+    }
+  }
+}
+```