Add Romulus to examples in README

Recent documents use Romulus as the example platform for
getting developers up to speed on developing for OpenBMC:

https://github.com/openbmc/docs/tree/master/development



Romulus was used because it's used by CI for every job,
therefore being the most stable and supported.

Change-Id: Icaac937696305f6c9131b2063954cc523b6d2df5
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>