Add a sample to clarify animation callback

Summary:
Even having worked long with React Native, I am pretty new to animations, so it took me a while to figure out how to trigger some code after an animation had finished. After some investigation, it
was evident I did not ready as in depth as I am supposed to, but this can be a problem for many
who, like me, try to search quick through docs and other resources.

Here more cases:
https://stackoverflow.com/questions/38053071/react-native-animated-complete-event
#3212

Read the proposed sample, and see if it is adequate.

 [DOCS] [ENHANCEMENT] [AnimatedImplementation.js] - Adds a sample for animation callbacks
Closes #16770

Differential Revision: D6304441

Pulled By: hramos

fbshipit-source-id: c22e391aece6a62684a78847fc74df203c2438d7

Author: jsdario

docs/animated.md

@@ -46,6 +46,16 @@ In most cases, you will be using `timing()`. By default, it uses a symmetric eas
 
 Animations are started by calling `start()` on your animation. `start()` takes a completion callback that will be called when the animation is done. If the animation finished running normally, the completion callback will be invoked with `{finished: true}`. If the animation is done because `stop()` was called on it before it could finish (e.g. because it was interrupted by a gesture or another animation), then it will receive `{finished: false}`.
 
+```javascript
+this.animateValue.spring({}).start(({finished}) => {
+  if (finished) {
+    console.log('Animation was completed')
+  } else {
+    console.log('Animation was aborted')
+  }
+})
+```
+
 ### Using the native driver
 
 By using the native driver, we send everything about the animation to native before starting the animation, allowing native code to perform the animation on the UI thread without having to go through the bridge on every frame. Once the animation has started, the JS thread can be blocked without affecting the animation.