Update readme

Author: Simon Brand

README.md

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 A lightweight non-owning reference to a callable.
 
+Clang + GCC: [![Linux Build Status](https://travis-ci.org/TartanLlama/function_ref.png?branch=master)](https://travis-ci.org/TartanLlama/function_ref)
+MSVC: [![Windows Build Status](https://ci.appveyor.com/api/projects/status/k5x00xa11y3s5wsg?svg=true)](https://ci.appveyor.com/project/TartanLlama/function_ref)
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 [![CC0](http://i.creativecommons.org/p/zero/1.0/88x31.png)]("http://creativecommons.org/publicdomain/zero/1.0/")