This commit updates the README to make it clearer that the CMSIS-RTOS2
FRI project is based on FRI. Descriptions of the difference between the
two projects and the purpose of CMSIS-RTOS2 FRI are added. Additional
documentation describing how further examples from FRI could be ported
to CMSIS-RTOS2 FRI is also added.

Signed-off-by: Chuyue Luo <Chuyue.Luo@arm.com>