prd: Add top level README.md explaining PRD

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
prd: Initial version of Persistent RAM Driver

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
prd: Add getgeo to block ops

Some programs require HDIO_GETGEO work, which requires we implement
getgeo.  Based off of the work done to the NVMe driver in this commit:

4cc09e2dc4cb NVMe: Add getgeo to block ops

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>