Documentation update for headless units

[zachjacobs]

I had issues running these programs on a headless Linux install because there was no frame buffer available.

I saw the error:

(ArduCam Demo:19080): Gtk-WARNING **: cannot open display:

To fix this, you can run a virtual frame buffer:

sudo apt-get install xvfb
sudo Xvfb :5 &
export DISPLAY=:5
sudo python ArduCam_Py_Demo.py ../../../python_config/MT9F001_14MP.json

It might be worth it to update your documentation.