Update README.md

Add coverity badge

Author: righettod

README.md

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+[![Build Status](https://travis-ci.org/righettod/pst-digger.svg?branch=master)](https://travis-ci.org/righettod/pst-digger)
+[![Coverity Status](https://scan.coverity.com/projects/13791/badge.svg)](https://scan.coverity.com/projects/righettod-pst-digger)
+
 # Description
 
 Program to analyze mails stored into a Microsoft Outlook PST file and find one based on search keywords.
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 The program will be build as the file **target/pst-digger.jar**
 
-# Travis-CI build status
-
-[![Build Status](https://travis-ci.org/righettod/pst-digger.svg?branch=master)](https://travis-ci.org/righettod/pst-digger)
-
 # Running requirement
 
 The program need a JRE 1.8