add deprecation guide for String prototype extensions

Author: locks

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

@@ -0,0 +1,52 @@
+---
+id: ember-string.prototype_extensions
+title: Deprecate String prototype extensions
+until: '4.0.0'
+since: 'Upcoming Features'
+---
+
+Ember applications add methods to the `String` prototype by default, making you able to call `"Tomster".dasherize()`, for example.
+Calling [these methods](https://api.emberjs.com/ember/3.22/classes/String) on a string itself should be replaced with importing 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
+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 might want to replace these methods with another library, like [lodash](https://lodash.com/).
+Keep in mind that different libraries will behave in slightly different ways, so make sure any critical transformation is 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,
+    }
+  }
+}
+```