docs: adding the npm package version badge to README.md (#18)

Author: Hesam Bahrami

README.md

@@ -1,5 +1,7 @@
 # Nested Sort
 
+[![npm version](https://badge.fury.io/js/nested-sort.svg)](https://badge.fury.io/js/nested-sort)
+
 Nested Sort is a vanilla JavaScript library, without any dependencies, which helps you to sort a nested list of items via drag and drop. Unfortunately, it does not support touch screens yet.
 
 ![](demo.gif)