Add "Available on the Unity Asset Store" button

Author: lopespm

README.md

@@ -2,13 +2,15 @@
 
 Concise Unity library which dynamically keeps a set of objects (e.g. players and important objects) in view, a common problem for a wide range of games. 
 
-The library was developed for, and used by my first Steam game, [Survival Ball](https://survivalball.com/). The game has an heavy shared screen local co-op component, which requires the camera to dynamically keep many key elements in view.
+The library was originally developed for, and used by [Survival Ball](https://survivalball.com/), a game with a heavy shared screen local co-op component, requiring the camera to dynamically keep many key elements in view. More information about the library's inner workings and underlying math in the related [blog article](https://lopespm.github.io/libraries/games/2018/12/27/camera-multi-target.html).
 
-More information about the library's inner workings and underlying math is available in the related [blog article](https://lopespm.github.io/libraries/games/2018/12/27/camera-multi-target.html)
+[<img src="https://user-images.githubusercontent.com/3640622/71978096-714ea100-3212-11ea-9480-cbc0ce33df4a.png"
+      alt="Get it for free on the Unity Asset Store"
+      height="80">](https://assetstore.unity.com/packages/tools/camera/camera-multi-target-dynamic-135922)
 
-<img src="https://user-images.githubusercontent.com/3640622/50427667-19165c80-08a6-11e9-9306-6d05f7477262.jpg" alt="Camera Multi Target Example Scene Screenshot">
+[<img src="https://user-images.githubusercontent.com/3640622/50427650-c2108780-08a5-11e9-9db0-d69e9fa329c1.gif" alt="Camera Multi Target Example Scene Video">](https://www.youtube.com/watch?v=In3eVapQ5mk)
 
-[<img src="https://user-images.githubusercontent.com/3640622/50427650-c2108780-08a5-11e9-9db0-d69e9fa329c1.gif" alt="Camera Multi Target Example Scene Video">](https://www.youtube.com/watch?v=In3eVapQ5mk) 
+<img src="https://user-images.githubusercontent.com/3640622/50427667-19165c80-08a6-11e9-9306-6d05f7477262.jpg" alt="Camera Multi Target Example Scene Screenshot">
 
 
 ## Install
@@ -76,4 +78,4 @@ An example scene of the library's usage is included in this repository and in th
 
 - [Survival Ball](https://survivalball.com/)
 
-*Did you develop, or know about, a game using this library? Add it here via [pull request](https://github.com/lopespm/unity-camera-multi-target/pulls)*
+*Did you develop, or know about, a game using this library? Add it here via [pull request](https://github.com/lopespm/unity-camera-multi-target/pulls)*