Rename platformio_override.ini to platformio_override_sample.ini

[schlimmchen]

this file is supposed to be changed by users, but users' changes shall never be included in the upstream project. also, it is cumbersome that git marks the file as changed and that it has to be cleaned up when switching branched etc.

rename the file to platformio_override_sample.ini. this way users who actually want to use the platformio_override.ini have something to start from. they only need to copy the sample file. using '_sample' within the file name rather than manipulating the file ending is good for windows users who might not be displaying file extensions in their file explorer.

[gregorwolf]

Beside providing the '_sample' I would suggest adjusting the README.md so the documentation is in sync with the files.

[schlimmchen]

Good point. Unfortunately, I was having a hard time to incorporate this information without changing the structure of the README. That is because the platformio_override.ini file was mentioned in multiple locations. So... I may have gone a little too far, but I hope that the proposed changes are an overall improvement to the README.

[schlimmchen]

Rebased once more onto the current master, in case @tbnobody wants to consider this change. Adjusted the respective commit message and the title of this PR to reflect what they actually achieve.

Renaming the platformio_override.ini file solves the problem that changes to that file will be tracked regardless of the fact that the file is mentioned in .gitignore. I assume that was the intention when adding the file to .gitignore originally.