README: Update for recent changes
README: Update for recent changes

- Update REQUIREMENTS section for new VMM drivers, and move VMM-specific
dependencies into their own subsections

- Add section listing and describing available VMM drivers

- Discuss libvdisk in the context of the vmware driver rather than as an
independent component

- Bring CONFIGURE OPTIONS section up-to-date with current configure
options

- Update INSTALLATION PATHS and POST-INSTALLATION sections for module
build changes

- Update license section with information on VMM drivers

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