btrfs_doc: add questions to btrfs transaction concepts

Signed-off-by: Wang Shilong <wangshilong1991@gmail.com>
btrfs_doc: snapshot concepts

Signed-off-by: Wang Shilong <wangshilong1991@gmail.com>
btrfs_doc: update readme

Signed-off-by: Wang Shilong <wangshilong1991@gmail.com>