add pcg license file to where pcg is integrated

Author: abeham

src/SimSharp/Random/LICENSE-PCG.txt

@@ -0,0 +1,13 @@
+MIT License
+
+Copyright (c) 2016 Bismur Studios Ltd.
+Copyright (c) 2016 Ioannis Giagkiozis
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software 
+and associated documentation files (the "Software"), to deal in the Software without restriction, 
+including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, 
+and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, 
+subject to the following conditions:
+
+ The above copyright notice and this permission notice shall be included in all copies or substantial 
+ portions of the Software.