add readme for program trace directory

c64/program_traces/README.md

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+Program traces are generated by running VICE's C64 emuluator (x64) with
+the collect_data program, to generate the list of KERNAL calls that a
+program makes.  This is used to generate the set of entry points that
+are required in order to provide interoperability with the original
+C64 KERNAL.
+
+To collect a program trace:
+
+x64 -remotemonitor &
+src/collect_data localhost 6510 | tee logfile
+
+