Update README.md

 - added TosaSerializationRegion
 - aligned API's in each class with current source code
 - removed reference to include/operator.def (do not exist)
 - removed reference to quantization information (no long applies)

Signed-off-by: Tai Ly <tai.ly@arm.com>
Change-Id: I33e83284e42a15a508f39ed0ebeef6e4db479568