V18: docs: document for add-cow file format

Document for add-cow format, the usage and spec of add-cow are
introduced.

V17->V18:
1) remove version field.
2) header size is maximum value and cluster size value.
3) fix type.

Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
V19: docs