Update top level README
Update top level README

Update the top level README file for the following API changes:
* interrupt handling added for generic and FreeRTOS
* condition variable
* spinlock
* cache operations.

Signed-off-by: Wendy Liang <cac46fad14f88489ab4dbc3c1082c7dafae20187@xilinx.com>

forg0ne/libmetal,forg0ne/libmetal