README: clarify when RLIMIT_MEMLOCK is still used
README: clarify when RLIMIT_MEMLOCK is still used

For newer kernels, the rings themselves are not accounted via
RLIMIT_MEMLOCK, but registered buffers still are.

Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing