Document the version of the platform library to be used

[dmohns]

It's not clear which version of the platform library is used. Latest is 2.4.0

Also, it looks like we are using some deprecated version:

ID                       Version Name                  
STM32:stm32              1.9.0   STM32 Cores           
STMicroelectronics:stm32 2.4.0   STM32 MCU based boards
STMicroelectronics:stm8  1.0.0   STM8 MCU based boards

Note the different name schema.

Also, document which board and board part have to be used. From the screenshots it looks like Generic STM32F1 series. However, in default setting this doesn't work, as the default has too small memory.

From some quick testing it looks like the correct FQBN is

STM32:stm32:GenF1:pnum=BLUEPILL_F103C8

[dmohns]

This is now done via platformio.ini pinning the platform = ststm32@13.0.0