Add CI badge

Author: orleika

README.md

@@ -1,6 +1,7 @@
 # docker-nginx
 
 [![MIT License](http://img.shields.io/badge/license-MIT-blue.svg?style=flat-square)](https://orleika.github.io/mit-license)
+[![Build Status](https://img.shields.io/travis/orleika/docker-nginx/master.svg?style=flat-square)](https://travis-ci.org/orleika/docker-nginx)
 [![nginx](http://img.shields.io/badge/nginx-v1.11.7-blue.svg?style=flat-square)](https://nginx.org/en/download.html)
 [![LibreSSL](http://img.shields.io/badge/LibreSSL-v2.4.4-blue.svg?style=flat-square)](https://www.libressl.org/)