README: add that this also applies for 3.5 kernels

Signed-off-by: Alejandro Martinez Ruiz <alex@nowcomputing.com>
f2fs-for-3.8: add patch fixing discard for large requests

Signed-off-by: Alejandro Martinez Ruiz <alex@nowcomputing.com>
README: add instructions for 3.8 kernels

Signed-off-by: Alejandro Martinez Ruiz <alex@nowcomputing.com>