TFLM: added optimized kernel directory for vexriscv

tensorflow/lite/micro/kernels/vexriscv/README.md

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+# VexRISC-V
+## Maintainers
+* [danielyou0230](https://github.com/danielyou0230)
+* [tal-x](https://github.com/tcal-x)
+
+## Background
+The optimized kernels for [VexRISC-V](https://github.com/SpinalHDL/VexRiscv)/[Litex](https://github.com/enjoy-digital/litex) are used to run Tensorflow Lite Micro in Zephyr on either
+
+* Digilent Arty board (e.g. Arty A7)
+* [Renode](https://github.com/renode/renode): Open source simulation framework (no hardware required)
+
+To run on Digilent Arty board (FPGA,) you'll also need a soft-CPU gateware for the FPGA, please see [Tensorflow lite demo running in Zephyr on Litex/VexRiscv SoC](https://github.com/antmicro/litex-vexriscv-tensorflow-lite-demo) by Antmicro for more details.
+
+## Info
+To use VexRISC-V optimized kernels instead of reference kernel add `TAGS=vexriscv`
+to the make command. The kernels that doesn't have optimization for a certain micro
+architecture fallback to use TFLM reference kernels.
+
+# Example
+
+To compile the binary file with VexRISC-V optimizations, one can use the following command
+
+```
+make -f tensorflow/lite/micro/tools/make/Makefile \
+TAGS=vexriscv \
+TARGET=zephyr_vexriscv \
+person_detection_int8_bin
+```