add CDN use option with unpkg (#1844)

closes #1507

Author: Luis Romero

README.md

@@ -66,6 +66,12 @@ The library can also be installed through [bower][bower]
 $ bower install validator-js
 ```
 
+CDN
+
+```html
+<script src="https://unpkg.com/validator@latest/validator.min.js"></script>
+```
+
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