Update README.md

Added link to online Doxygen at the beginning of the readme

Author: jdsika

README.md

@@ -3,6 +3,7 @@ Open Simulation Interface (OSI)
 
 [![Travis Build Status](https://travis-ci.org/OpenSimulationInterface/open-simulation-interface.svg?branch=master)](https://travis-ci.org/OpenSimulationInterface/open-simulation-interface)
 
+[Online Doxygen Documentation](https://opensimulationinterface.github.io/open-simulation-interface/)
 
 General description
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