Add PL/pg SQL (without test support)

plpgsql/README.md

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+# Setup
+Create database structure: psql -f ./structure/create.sql
+Load code into database psql -f ./code/update_quality.sql -d gilded_rose
+Load test data into database: psql -f ./test/data/load.sql -d gilded_rose
+Execute: CALL update_quality();
+Check results: SELECT * FROM items;
+
+# Test
+No test code provided, only a sample of test data
+TODO: [https://pgtap.org/](introduce TAP - xUnit for PL/pg SQL)
+