disas: nanoMIPS: Add a note on documentation

Add "nanoMIPS32 Instruction Set Technical Reference Manual" as
a reference.

Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>

disas/nanomips.cpp

@@ -20,6 +20,13 @@
  *
  */
+/*
+ *  Documentation used while implementing this component:
+ *
+ *  [1] "MIPS® Architecture Base: nanoMIPS32(tm) Instruction Set Technical
+ *      Reference Manual", Revision 01.01, April 27, 2018
+ */
+
 extern "C" {
 #include "qemu/osdep.h"
 #include "disas/bfd.h"