Update README.md

README.md

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 In this repository, a new real gas model for hydrogen based on the REFPROP database is provided for OpenFOAM v2012. 
 
 
+## OpenFOAM folder structure
+
+![](OpenFOAM_folder_structure.pdf)
+
+
 ## Tutorial
 
 In a tutorial case, the usage of the new real gas model is shown.