PSA Add README to psa/examples/historical directory

p4-16/psa/examples/historical/README.txt

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+Files with 'value-sets' in their name:
+
+Parser value sets were originally considered to be defined as an
+extern object in PSA, but all of the ways discussed were felt to be
+too unwieldy, either for the P4 developer, the P4 compiler developers,
+or both, vs. making parser value sets a new P4_16 language construct
+defined in the language specification, similar to (but simpler than)
+tables.  That happened in early 2018.  All files with 'value-sets' as
+part of their name represent approaches considered for making them an
+extern in PSA.