bitpit: add Coverity status badge in Readme.md

Author: marcocisternino

README.md

@@ -1,4 +1,8 @@
 [![Build Status](https://www.travis-ci.org/optimad/bitpit.svg?branch=travis)](https://www.travis-ci.org/optimad/bitpit)
+<a href="https://scan.coverity.com/projects/optimad-bitpit">
+  <img alt="Coverity Scan Build Status"
+       src="https://scan.coverity.com/projects/13132/badge.svg"/>
+</a>
 
 bitpit is a C++ library for scientific High Performance Computing.
 Within bitpit different modules factorize the typical effort which is needed to derived a real-life application code.