add comment pointing to postgres code

flow/hstore/hstore.go

@@ -1,3 +1,9 @@
+/*
+This is in reference to PostgreSQL's hstore:
+https://github.com/postgres/postgres/blob/bea18b1c949145ba2ca79d4765dba3cc9494a480/contrib/hstore/hstore_io.c
+
+This package is an implementation of the above code.
+*/
 package hstore_util
 
 import (