Add Coverity Scan status badge

README.md

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 [![Build Status](https://travis-ci.org/vcmi/vcmi.svg?branch=develop)](https://travis-ci.org/vcmi/vcmi)
+[![Coverity Scan Build Status](https://scan.coverity.com/projects/vcmi/badge.svg)](https://scan.coverity.com/projects/vcmi)
 # VCMI Project
 VCMI is work-in-progress attempt to recreate engine for Heroes III, giving it new and extended possibilities. To use VCMI you need to own original data files.