Add a README to the board which lists the commands required to enable
booting from the eMMC boot partitions found on the board.

Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: NTom Rini <trini@ti.com>
Signed-off-by: NPantelis Antoniou <panto@antoniou-consulting.com>