README.md: show the coverall coverage on github

README.md

@@ -2,6 +2,7 @@
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/63/badge)](https://bestpractices.coreinfrastructure.org/projects/63)
 [![Coverity passed](https://scan.coverity.com/projects/curl/badge.svg)](https://scan.coverity.com/projects/curl)
 [![Build Status](https://travis-ci.org/curl/curl.svg?branch=master)](https://travis-ci.org/curl/curl)
+[![Coverage Status](https://coveralls.io/repos/github/curl/curl/badge.svg)](https://coveralls.io/github/curl/curl)
 
 Curl is a command-line tool for transferring data specified with URL
 syntax. Find out how to use curl by reading [the curl.1 man