Add supported features list to README

People have been asking for a list of things OpenBMC
supports.  This is a list of operator useable features
along with items that are in progress and additional
features we are looking for support.

Change-Id: I58ceb6aa32bbf68c190ba0eac5419979fbe9aa63
Signed-off-by: Chris Austen <austenc@us.ibm.com>