Add a citation to Seltzer and Yigit's Usenix '91 paper about hash table

Add a citation to Seltzer and Yigit's Usenix '91 paper about hash table
management.  The paper clearly describes many of the ideas embodied in
our current hashing code, but as far as I could find out there is not
a direct code heritage.  (Mike Olsen recalls discussion of this paper
at Postgres meetings but believes it "informed the Postgres implementation
probably just at the design level".  Margo herself says she wasn't
involved with Postgres' hash code.)  Credit where credit is due 'n all
that, even if fifteen years after the fact.