Create README.md

Author: fpistm

src/genpinmap/README.md

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+# genpinmap
+
+This script is able to generate the `PeripheralPins.c` for a specific board.
+
+After file generation, review it carefully and please report any issue
+[here](https://github.com/stm32duino/Arduino_Tools/issues)
+
+Once generated, you should comment a line if the pin is generated<br>
+several times for the same IP or if the pin should not be used<br>
+(overlaid with some HW on the board, for instance)
+
+Usage: `python genpinmap_arduino.py <Name> <product xml file name>`
+   - `<Name>` is the name of the directory where `PeripheralPins.c` will be generated under _./Arduino_ directory
+   - `<product xml file name>` is the STM32 file description in [STM32CubeMX](http://www.st.com/en/development-tools/stm32cubemx.html)
+
+**!! This xml file contains non alpha characters in its name, you should call it with quotes**