Added coverage badge

Author: Takashi Matsuo

README.md

@@ -3,6 +3,7 @@
 A collection of samples that demonstrate how to call Google Cloud services from PHP.
 
 [![Build Status](https://travis-ci.org/GoogleCloudPlatform/php-docs-samples.svg)](https://travis-ci.org/GoogleCloudPlatform/php-docs-samples)
+[![Coverage Status](https://coveralls.io/repos/github/GoogleCloudPlatform/php-docs-samples/badge.svg?branch=master)](https://coveralls.io/github/GoogleCloudPlatform/php-docs-samples?branch=master)
 
 See our other [Google Cloud Platform github
 repos](https://github.com/GoogleCloudPlatform) for sample applications and