update readme with RAST docs location
this script can be used to get RAST annotations for subsystems

it must be run like all the other scripts in vendor/sas/plbin, but
if you pipe through a bindings file from the Subsystem/ subdirectory
of a tarball exported with svr_retrieve_RAST_job which is documented
at http://pubseed.theseed.org/sapling/server.cgi?pod=RASTserver then
it will add columns for the Category and Subcategory