README: Add Renesas RA port information.

* Add Renesas RA port information
* Add short explanation about installation instructions.

Signed-off-by: Takeo Takahashi <[email protected]>

Author: TakeoTakahashi2020

README.md

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+MicroPython for Renesas microcontrollers
+========================================
+* `Renesas RA port` for Renesas RA Family microcontroller is supported and released by [the MicroPython project](https://github.com/micropython/micropython) since 29 April, 2022.
+* The latest information about Renesas RA port is available on [micropython.org](https://micropython.org). 
+* Please refer to the [Quick reference for the Renesas RA](https://docs.micropython.org/en/latest/renesas-ra/quickref.html) in [MicroPython documentation](https://docs.micropython.org/en/latest) about the features of Renesas RA port.
+* Please refer to [Renesas RA port](https://docs.micropython.org/en/latest/renesas-ra/general.html) about supported boards.
+* The pre-built firmware image is available on each board page of [DOWNLOAD](https://micropython.org/download/?port=renesas-ra). This page also includes installation instructions.
+* If you are interested in building and debugging, please refer to the Wiki pages [How to build](https://github.com/renesas/micropython/wiki/How-to-build) and [How to debug](https://github.com/renesas/micropython/wiki/How-to-debug).
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+<BR>
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