README.md: add Coverity Badge

Add badge reporting the status of the Coverity at the top of the README
file.

Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>

Author: gcabiddu

README.md

@@ -1,5 +1,6 @@
 ![Linux build with gcc](https://github.com/intel/qatlib/actions/workflows/linux_build_gcc.yml/badge.svg)
 ![CodeQL scan](https://github.com/intel/qatlib/actions/workflows/codeql.yml/badge.svg)
+[![Coverity Scan Build Status](https://scan.coverity.com/projects/intel-qatlib/badge.svg)](https://scan.coverity.com/projects/intel-qatlib)
 [![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/intel/qatlib/badge)](https://scorecard.dev/viewer/?uri=github.com/intel/qatlib)
 
 # Intel® QuickAssist Technology Library (QATlib)