README: add calling contract details
README: add calling contract details

Provide contract details for what handlers can expect from the blob
manager.  This saves extra checks within the handler.

Incidental markdown cleanup.

Signed-off-by: Patrick Venture <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>
Change-Id: I572fdfb9e726d9f4d1ba89bb92b38c67cd7f07e2

openbmc/phosphor-ipmi-blobs