README: Add span-lite install
README: Add span-lite install

Change-Id: I77284753d11ee7c845f4a0535a47dc70a3beed48
Signed-off-by: William A. Kennington III <30fff5198f4d10ca1117e5f3ec82f3b4b8900d81@google.com>

openbmc/phosphor-ipmi-flash