Add Apache 2.0 License badge to README. (#239)

diff --git a/README.md b/README.md
index 1306ebe..9cdfb36 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,7 @@
 OpenWhisk API Gateway
 =============
 [![Build Status](https://travis-ci.org/apache/incubator-openwhisk-apigateway.svg?branch=master)](https://travis-ci.org/apache/incubator-openwhisk-apigateway)
+[![License](https://img.shields.io/badge/license-Apache--2.0-blue.svg)](http://www.apache.org/licenses/LICENSE-2.0)
 
 A performant API Gateway based on Openresty and NGINX.