Add package description.

Author: ChebanovDD

src/UnityMvvmToolkit.UnityPackage/Assets/Plugins/UnityMvvmToolkit/package.json

@@ -4,7 +4,7 @@
     "author": { "name": "ChebanovDD", "url": "https://github.com/ChebanovDD" },
     "version": "0.1.0",
     "unity": "2018.4",
-    "description": "...",
+    "description": "The Unity Mvvm Toolkit allows you to use data binding to establish a connection between the app UI and the data it displays. This is a simple and consistent way to achieve clean separation of business logic from UI.",
     "keywords": [ "mvvm", "binding", "ui", "toolkit" ],
     "license": "MIT"
 }