NFSD: Add instructions on how to deal with xdrgen files

NFSD: Add instructions on how to deal with xdrgen files

xdrgen requires a number of Python packages on the build system. We
don't want to add these to the kernel build dependency list, which
is long enough already.

The generated files are generated manually using

  $ cd fs/nfsd && make xdrgen

whenever the .x files are modified, then they are checked into the
kernel repo so others do not need to rebuild them.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>