Update PlatformIO.md serialUPDI instructions

[Elon61]

Added a bit more information in regards to using SerialUPDI in PlatformIO since i struggled a little getting it to work.

The custom method was inspired by Henry Cheung's blog post, and allows you to use this core's uploader with very little hassle, but i would suggest simply using the serialupdi upload protocol which worked well enough for my purposes.