Add a note about the PRG button

[glennra]

Your repo has been a helpful resource for this board, thanks. Today I wanted to use the PRG button and couldn't find any information specifically about reading it from code. I could see from the pinout diagram that it's supposed to be connected to GPIO 0 but couldn't get it to work. Turns out that you need to enable the internal pull up. It would have saved me a an hour or so if that info was to hand and your repo seems like an appropriate place for it.