docs(ngAnimate): add docs for the usage of the ng-animate CSS class

Author: matsko

src/ngAnimate/module.js

@@ -224,6 +224,35 @@
  *
  * Stagger animations are currently only supported within CSS-defined animations.
  *
+ * ### The `ng-animate` CSS class
+ *
+ * When ngAnimate is animating an element it will apply the `ng-animate` CSS class to the element for the duration of the animation.
+ * This is a temporary CSS class and it will be removed once the animation is over (for both JavaScript and CSS-based animations).
+ *
+ * Therefore, animations can be applied to an element using this temporary class directly via CSS.
+ *
+ * ```css
+ * .zipper.ng-animate {
+ *   transition:0.5s linear all;
+ * }
+ * .zipper.ng-enter {
+ *   opacity:0;
+ * }
+ * .zipper.ng-enter.ng-enter-active {
+ *   opacity:1;
+ * }
+ * .zipper.ng-leave {
+ *   opacity:1;
+ * }
+ * .zipper.ng-leave.ng-leave-active {
+ *   opacity:0;
+ * }
+ * ```
+ *
+ * (Note that the `ng-animate` CSS class is reserved and it cannot be applied on an element directly since ngAnimate will always remove
+ * the CSS class once an animation has completed.)
+ *
+ *
  * ## JavaScript-based Animations
  *
  * ngAnimate also allows for animations to be consumed by JavaScript code. The approach is similar to CSS-based animations (where there is a shared