Add note about macOS/BSD serial device paths

Hopefully this'll save others some time; it took a little bit of research to figure it out myself. (Aside from the device names, everything seems to work the same on macOS as on Linux)

I don't think it's worth mentioning in the README, since it should work either way, but for any other Mac users who might stumble across this: If you had to install a USB driver once upon a time to get some Arduino clone to work with your Mac, you can get rid of it now (delete the file from /Library/Extensions). Apple added CH34x support to their own driver in Mojave and Catalina