update README

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
DRAFT: modified writeboost.txt

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
moving discard from create-vol.sh to cleanup-cache.sh

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>