added a query optimization reference (optional) to readings list

[Subbarao Kambhampati <SUBBARAO.KAMBHAMPATI@asu.edu>]

Since I went into more detail than I expected in discussing query
optimization, it is possible that some of you might be interested in
getting a good low-down on query optimization in relational
databases. A quite up-to-date tutorial on query optimization by
Surajit Chaudhury given at PODS-98 has been added to the readings
list. The paper is of course optional. 

It may come as no surprise that my own research interests, as far as
this course is concerned, are in query optimization in the context of
information integration. 

Rao