Add coverity badge

Coverity now scans our JS code…

README.md

@@ -9,6 +9,7 @@ Git master: [![Build Status](https://ci.owncloud.org/job/server-master-linux/bad
 Quality: 
  - Scrutinizer: [![Scrutinizer Quality Score](https://scrutinizer-ci.com/g/owncloud/core/badges/quality-score.png?s=ce2f5ded03d4ac628e9ee5c767243fa7412e644f)](https://scrutinizer-ci.com/g/owncloud/core/)
  - CodeClimate: [![Code Climate](https://codeclimate.com/github/owncloud/core/badges/gpa.svg)](https://codeclimate.com/github/owncloud/core)
+ - Coverity: [![Coverity](https://scan.coverity.com/projects/6893/badge.svg)](https://scan.coverity.com/projects/owncloud-core)
 
 Dependencies: