Added some badges to the Readme.

README.md

 OpenGeoSys 6                                                      {#mainpage}
 ============
 
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 [OpenGeoSys][ogs] (OGS) is a scientific open source project for the development of
 numerical methods for the simulation of thermo-hydro-mechanical-chemical
 (THMC) processes in porous and fractured media. OGS is implemented in C++, it