README.md: restore and fix code coverage badge

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/codec/trunk@1812001 13f79535-47bb-0310-9956-ffa450edef68

Author: PascalSchumacher

README.md

@@ -44,6 +44,7 @@ Apache Commons Codec
 ===================
 
 [![Build Status](https://travis-ci.org/apache/commons-codec.svg?branch=trunk)](https://travis-ci.org/apache/commons-codec)
+[![Coverage Status](https://coveralls.io/repos/apache/commons-codec/badge.svg?branch=trunk)](https://coveralls.io/r/apache/commons-codec)
 
 The Apache Commons Codec package contains simple encoder and decoders for
      various formats such as Base64 and Hexadecimal.  In addition to these