README: add npm badge (#39)

Signed-off-by: Stephan Renatus <stephan@styra.com>

Author: srenatus

README.md

@@ -3,6 +3,7 @@
 Styra's OPA SDK
 
 [![License](https://img.shields.io/badge/License-Apache_2.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
+[![NPM Version](https://img.shields.io/npm/v/%40styra%2Fopa?style=flat&color=%2324b6e0)](https://www.npmjs.com/package/@styra/opa)
 
 <!-- Start SDK Installation [installation] -->
 ## SDK Installation