Few mistakes in README.md

[miniwark]

I am testing to ... blink a LED and i was able to do so, but the README.md file have a few little errors who may confuse new users:

1. the pip install command is now:

pip install arduino-python

2. In the blink a led example, the digital pin must be set to OUTPUT before the blink loop:

board.pinMode(13, "OUTPUT")

3. The fact than prototype.ino must be uploaded with Arduino IDE before launching the python script may be clarified a bit more.