docs: add information about the utility of this lib into the README + pictures + stackblitz demo

Author: maxime1992

README.MD

@@ -1,4 +1,22 @@
 # ngx-hover-opacity
+This librairy aims to provide a simple way to "disable" an entire HTML element. 
+
+For example, if you display a given resource and suddenly it is not available anymore (deleted on the server and you just received an event).
+Should you redirect the user to the home page? If he was reading it, it's a little brutal, isn't it?
+
+Instead of that, you might want to take a breath and do not remove the element right away.
+Just hover it, with a nice opacity so the user cannot interact with it anymore.
+(Maybe you could also let him know what's happening with a message, but it's up to you).
+
+## Demo: 
+
+ngx-hover-opacity set to **OFF**  
+![An MdCard from material with ngx-hover-opacity set to OFF](https://user-images.githubusercontent.com/4950209/29740631-2ee2ca28-8a5b-11e7-9f61-19681ab07fbd.png)
+
+ngx-hover-opacity set to **ON**  
+![An MdCard from material with ngx-hover-opacity set to ON](https://user-images.githubusercontent.com/4950209/29740633-4446d972-8a5b-11e7-9c61-f231d40d803a.png)
+
+You can also [try it live on Stackblitz!](https://stackblitz.com/edit/angular-se17ec)
 
 ## Installation