Move most /contrib README files into SGML. Some still need conversion

contrib/adminpack/README.adminpack

contrib/btree_gist/README.btree_gist

contrib/chkpass/README.chkpass

contrib/cube/README.cube

contrib/dblink/README.dblink

contrib/earthdistance/README.earthdistance

contrib/fuzzystrmatch/README.fuzzystrmatch

contrib/hstore/README.hstore

contrib/intagg/README.int_aggregate

contrib/intarray/README.intarray

contrib/isn/README.isn

contrib/lo/README.lo

contrib/ltree/README.ltree

contrib/pg_standby/README.pg_standby

contrib/pg_trgm/README.pg_trgm

contrib/pgbench/README.pgbench

contrib/pgcrypto/README.pgcrypto

contrib/pgrowlocks/README.pgrowlocks

contrib/pgstattuple/README.pgstattuple

contrib/seg/README.seg