document many more cooincidental similarities with C64 ROMs

strings/DD60AD00DD29

@@ -0,0 +1,7 @@
+do something with $DD00, then RTS, then read $DD00
+
+Again, $DD00 is a CIA IO register, and so there are a number 
+of routines that are required that simply check or modify the
+contents of this register in order to implement the IEC serial
+protocol.  Thus it is totally unsurprising to find routines that
+start by reading $DD00 and then processing it in some way.