da850: Add README.da850

This file documents when to build for da850evm and when to build for
da850_am18xxevm.  It also documents how to write the u-boot.ais file to
persistent storage (such as SPI), in some cases as well as how to write
a recovery image.

Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Christian Riesch <christian.riesch@omicron.at>
Acked-by: Prabhakar Lad <prabhakar.lad@ti.com>