message: Update Ethos-U documentation.
diff: <!-- mdformat off(b/169948621#comment2) -->
  # Info
  +Arm(R) Ethos(TM)-U is a new class of machine learning processors, called a
  +microNPU, specifically designed to accelerate ML inference in area-constrained
  +embedded and IoT devices. This readme briefly describes how to integrate Ethos-U
  +related hardware and software into TFLM.
  -To use Ethos-U kernel add CO_PROCESSOR=ethos_u to the make line. A tflite file
  -compiled by ARM's offline tool Vela is required for it to work. Armclang 6.14 is
  -required as compiler as well.
  +To enable the Ethos-U software stack, add `CO_PROCESSOR=ethos_u` to the make
  +command line. See example below.
  -## Vela example workflow
  +## Requirements:
  +- Armclang 6.14 or later
  +- GCC 10.2.1 or later
  +
  +## Ethos-U custom operator
  +The TFLM runtime will dispatch workloads to Ethos-U when it encounters an
  +Ethos-U custom op in the tflite file. The Ethos-U custom op is added by a tool
  +called Vela and contains information the Ethos-U hardware need to execute
  +the workload. More info in the [Vela repo](https://review.mlplatform.org/plugins/gitiles/ml/ethos-u/ethos-u-vela).
  ```
  | tensor0
  @@ -31,16 +42,19 @@ required as compiler as well.
  v
  ```
  -Note that ethousu_init() need to be called once during startup.
  +Note that the `ethousu_init()` API of the Ethos-U driver need to be called at
  +startup, before calling the TFLM API. More info in the [Ethos-U driver repo](https://review.mlplatform.org/plugins/gitiles/ml/ethos-u/ethos-u-core-driver).
  -(TODO: Add link to driver readme.) __FPU_PRESENT need to be set in target
  -makefile.
  +For even more info regarding Vela and Ethos-U, checkout [Ethos-U landing page](https://review.mlplatform.org/plugins/gitiles/ml/ethos-u/ethos-u/+/refs/heads/master).
  # Example 1
  -Compile a binary with Ethos-U kernel.
  +Compile a binary with Ethos-U support using the following command:
  ```
  make -f tensorflow/lite/micro/tools/make/Makefile network_tester_test CO_PROCESSOR=ethos_u \
  TARGET=<ethos_u_enabled_target> NETWORK_MODEL=<ethos_u_enabled_tflite>
  ```
  +
  +TODO: Replace `ethos_u_enabled_target` and `ethos_u_enabled_tflite` once the
  +Arm Corstone(TM)-300 example is up and running.
  