readme: update pinctrl instructions

Update pinctrl instructions to cover STM32 Open Pin Data changes.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

README.rst

@@ -112,13 +112,19 @@ Additionally, you need to add the following includes:
 ***********
 
 In order to provide STM32 boards pin configuration using device tree,
-*-pinctrl.dtsi are made available under dts/st directory.
-For each STM32 SoC package a complete and correct -pcintrl.dtsi is available.
-These files are generated using generation scripts available in this repo
-under scripts/genpinctrl. Generation use as input STM32 SoC description database
-delivered with ST STM32CubeMX official tool.
-New set of -pinctrl.dtsi files could be generated following availability of
-new version of STM32CubeMx or change in generation script configuration files.
+*-pinctrl.dtsi are made available under dts/st directory. For each STM32 SoC
+package a complete and correct -pcintrl.dtsi is available. Generation use as
+input the `STM32 Open Pin Data <https://github.com/STMicroelectronics/STM32_open_pin_data>`_
+database.
+
+*-pinctrl.dtsi files are generated using generation scripts available in this
+repo under scripts/genpinctrl and can be generated by running::
+
+   python3 scripts/genpinctrl/genpinctrl.py -p /path/to/stm32-open-pin-data-repository
+
+New set of -pinctrl.dtsi files could be generated following availability of a
+new version of STM32 Open Pin Data or a change in the generation script
+configuration files.
 
 STM32Cube based libraries:
 **************************