add singularity badge to readme

README.md

@@ -5,6 +5,8 @@
 
 [![](doc/source/images/agplv3.png)](COPYING)
 
+[![https://www.singularity-hub.org/static/img/hosted-singularity--hub-%23e32929.svg](https://www.singularity-hub.org/static/img/hosted-singularity--hub-%23e32929.svg)](https://singularity-hub.org/collections/4724)
+
 This README file provides some essential information to install/use PanACoTA. But it is better to read the [**full documentation**](http://aperrin.pages.pasteur.fr/pipeline_annotation/html-doc), providing more details : [![](doc/source/images/manual.jpg) ](http://aperrin.pages.pasteur.fr/pipeline_annotation/html-doc)
 
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