README: Improve clarity of the opening description

"can be described as" sounds a lot like Chris Austen having a chat with
someone over a beer. To the extent that Yocto is a Linux distribution
(which it isn't, but that's somewhat of a detail), OpenBMC is also a
distribution.

Change-Id: I5e72c3b357441a41c525b843bc1768d5946b645b
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>