some missing infos for ALSA config

[benjaminaigner]

Currently, following information is missing in the "Installation" docs:

• Comment out the line
  options snd-usb-audio index=-2
  in /lib/modprobe.d/aliases.conf
• Select the output (jack plug or HDMI) via
  sudo raspi-config
  BEFORE rebooting with the new ALSA settings (jasper.conf)

If you do the steps currently described, it is not possible afterwards to select the audio output with raspiconfig.