CMSIS-NN: Add information on legacy and TFL micro compliant APIs
CMSIS-NN: Add information on legacy and TFL micro compliant APIs

Information is added on the key differences between the APIs that
support TFL micro and the legacy APIs

Change-Id: Ic56fb85d778a9f3d42aeb0b7a107bd835dd37cfd

ARM-software/CMSIS_5,ARM-software/CMSIS_5,JonatanAntoni/CMSIS_5,JonatanAntoni/CMSIS_5,ARM-software/CMSIS_5,ARM-software/CMSIS_5,ARM-software/CMSIS_5,JonatanAntoni/CMSIS_5,ARM-software/CMSIS_5,JonatanAntoni/CMSIS_5,JonatanAntoni/CMSIS_5,JonatanAntoni/CMSIS_5