ENH: provided reference for Murphy-Shaddix coal char combustion model

src/lagrangian/coalCombustion/submodels/surfaceReactionModel/COxidationMurphyShaddix/COxidationMurphyShaddix.H

@@ -28,6 +28,11 @@ Class
 Description
     Limited to C(s) + O2 -> CO2
 
+    Loosely based on the reference:
+        Murphy, J. J., Shaddix, C. R., Combustion kinetics of coal chars
+        in oxygen-enriched environments, Combustion and Flame 144,
+        pp710-729, 2006
+
 \*---------------------------------------------------------------------------*/
 
 #ifndef COxidationMurphyShaddix_H