Add event log section to README
Add event log section to README

Describe what event logs are how and how to create them from
code.

Signed-off-by: Matt Spinler <c000c989d08c98662d98f15448f11de169feb92d@us.ibm.com>
Change-Id: I3f849a46a7fac1edb5965e916899c4a04333f6ca

openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging