Added comment about 9 entries in permeability tensor.

opm/core/props/rock/RockFromDeck.cpp

@@ -74,6 +74,7 @@ namespace Opm
 
     RockFromDeck::RockFromDeck(std::size_t number_of_cells)
         : porosity_(number_of_cells, 0),
+          // full permeability tensor in 3D stores 9 scalars
           permeability_(number_of_cells*9, 0.0),
           permfield_valid_(number_of_cells, false)
     {