README.server: Document parcel formats and upgrade command
README.server: Document parcel formats and upgrade command

- Document change to default parcel format, format override mechanisms,
and isr_admin upgrade

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr