[doc] added doc for internal var initialization

Documentation/ProjectFile/material/solid/constitutive_relation/MFront/c_MFront.md

@@ -4,3 +4,15 @@ with MFront's generic interface.
 See http://tfel.sourceforge.net/ https://github.com/thelfer/MFrontGenericInterfaceSupport
 
 This requires MFront 3.2.0 or newer (because of MFront's "generic" interface).
+
+\attention
+OpenGeoSys initializes the internal state of the MFront material model to
+all-zero. Furthermore, the very first iteration of OpenGeoSys is performed
+already at the first load step, i.e., OpenGeoSys does not perform a stress
+integration with the IC values. That differs, e.g., from the mtest executable
+shipped with MFront, which, initially does a stress integration with the ICs
+values. If your model needs such a stress integration step, you must make sure
+that you prepend an initial, artificial load step. If you observe convergence
+issues during the local stress integration in the very first iteration of
+OpenGeoSys, you should consider adding such a load step, in particular if mtest
+succeeds to integrate the same load path.