docs: Clarify relationship to PlatformIO in README.md

[brianredbeard]

Summary

Commit ab2e8a9 introduced the ability to consume this project as a library/package within the project PlatformIO.

This PR fixes/implements the following bugs/features

• Clarify use of Arduino_Core_STM32 with PlatformIO

Explain the motivation for making this change. What existing problem does the pull request solve?

This attempts to clarify (to the casual reader) where this Arduino core exists within the relationship between the projects.

Validation

N/A

Code formatting

• Ensure AStyle check is passed thanks CI

Closing issues

Affects #1368

[fpistm]

Hi @brianredbeard
thanks for this PR.
Anyway, I would not merge it.
This organization only support Arduino IDE. Adding those kind of information about PIO will let think that it is officially supported.
The commit you point was to ease PIO maintainer works and as this does not affect Arduino support we merge it.
I guess your documentation should be added to PIO docs.
If we want to clarify here in the README.md then this should be to tell that PIO is not supported by this organization and adding a link to PIO documenations.

[fpistm]

Hi @brianredbeard,

I've added a Wiki page based on your PR: https://github.com/stm32duino/wiki/wiki/PlatformIO
Do not hesitate to change it if you think it is not correct or could be better.
I don't know how the config is performed at PIO side, as stated before the commit you point was added by @valeros, I guess, to ease PIO maintainer works and as this does not affect Arduino support we merge it. Maybe I'm wrong or it should be updated?

Anyway I close this PR as I will not merge it.