Add code quality badge

Author: d-frey

README.md

@@ -5,6 +5,7 @@
 [![TravisCI](https://travis-ci.org/taocpp/operators.svg?branch=master)](https://travis-ci.org/taocpp/operators)
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 [The Art of C++](https://taocpp.github.io/) / Operators is a zero-dependency C++11 single-header library that provides highly efficient, move aware operators for arithmetic data types.