Installation Docs - updates

[HeneryH]

Two suggested changes

1. The apt installation of the rpi.gpio package has changed to the one noted below
2. The older node-red dashboard has been deprecated in favor of the one noted below.

sudo apt-get install -y build-essential nodejs nodered git-core rpi.gpio #python-rpi.gpio
cd ~/.node-red && npm install node-red-dashboard #node-red-contrib-ui