renesas-ra/README: Add basic details about board autogen files.

The renesas-ra port board definitions contain auto-generated files which
are not easy to understand for someone unfamiliar with the port.  This
commit adds some notes to the README to assist other developers who want to
work on these.

The configuration.xml file for the `ARDUINO_PORTENTA_C33` was provided by
@iabdalkader.

Signed-off-by: Andrew Leech <[email protected]>

ports/renesas-ra/README.md

@@ -63,6 +63,26 @@ passed as the argument to `BOARD=`; for example `RA4M1_CLICKER`, `EK_RA4M1`,
 The above command should produce binary images `firmware.hex` in the
 build-EK_RA6M2/` subdirectory (or the equivalent directory for the board specified).
 
+## Board definition auto-generated code
+
+The supported board definitions contain auto-generated configuration files in
+the `boards/<BOARD_NAME>/ra_cfg` and `boards/<BOARD_NAME>/ra_gen` folders.
+
+These are generated with the [RA Smart Configurator](https://www.renesas.com/us/en/software-tool/ra-smart-configurator)
+tool which is used to define peripheral configuration, pinouts, interrupts etc. for each board.
+
+This tool can be installed either as part of the "Renesas e² studio", or separately with
+the fsp driver package from https://github.com/renesas/fsp/releases eg.
+* [setup_fsp_v4_4_0_rasc_v2023-04.exe](https://github.com/renesas/fsp/releases/download/v4.4.0/setup_fsp_v4_4_0_rasc_v2023-04.exe)
+* [setup_fsp_v4_4_0_rasc_v2023-04.exe](https://github.com/renesas/fsp/releases/download/v4.4.0/setup_fsp_v4_4_0_rasc_v2023-04.AppImage)
+
+This tool can be used to create new board definitions or modify existing ones
+by opening one of the `configuration.xml` files in the board folders.
+
+Once the `configuration.xml` file is opened in RA Smart Configurator and modified as
+needed, the "Generate Project Content" button can be pressed to export new copies
+of the `ra_cfg` and `ra_gen` folders.
+
 ## Supported/Unsupported functions
 Please refer to the `renesas-ra` quick reference.