README.md : update gcc version

README.md

@@ -21,6 +21,12 @@ Install OpenFOAM, **any version should work** we have used:
 
 and related dependencies.
 
+**gcc version with support for the standard "--std=c++2a" is required**
+
+Tested with:
+
+* gcc (GCC) 9.1.0
+
 For the parameter variation, `PyFoam` is needed, it's available via `pip`
 
 ```