Add note for mt19937 source

pc/mt19937ar.h

@@ -1,3 +1,8 @@
+/* This header file provides prototypes for the Mersenne Twister
+   random number generator.  The corresponding mt19937.c is also
+   required and can be obtained from:
+     http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/MT2002/emt19937ar.html
+*/
 void init_genrand(unsigned long s);
 void init_by_array(unsigned long init_key[], int key_length);
 unsigned long genrand_int32(void);