Added {{did-insert}} API docs

Author: GavinJoyce

addon/modifiers/did-insert.js

@@ -1,5 +1,38 @@
 import Ember from 'ember';
 
+/**
+  The `{{did-insert}}` element modifier is activated when an element is
+  inserted into the DOM.
+
+  In this example, the `fadeIn` function receives the `div` DOM element as its
+  first argument and is executed after the element is inserted into the DOM.
+
+  ```handlebars
+  <div {{did-insert this.fadeIn}} class="alert">
+    {{yield}}
+  </div>
+  ```
+
+  ```js
+  export default Component.extend({
+    fadeIn(element) {
+      element.classList.add('fade-in');
+    }
+  });
+  ```
+
+  By default, the executed function won't be bound. If you would like to access
+  the component context in your function, use the `action` helper as follows:
+
+  ```handlebars
+  <div {{did-insert (action this.fadeIn)}} class="alert">
+    {{yield}}
+  </div>
+  ```
+
+  @method did-insert
+  @public
+*/
 export default Ember._setModifierManager(
   () => ({
     createModifier() {},