added codecov badge

Author: xrotwang

README.md

@@ -9,6 +9,7 @@ implements the methods described in the paper
 Note: `pyclics` requires python >=3.5
 
 [![Build Status](https://travis-ci.org/clics/clics2.svg?branch=master)](https://travis-ci.org/clics/clics2)
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 ## Command Line Interface