MQTT configuration for new home assistant

[masterx1981]

Hi!
The documentation must be updated for the new versions of home assistant.

Instead of:
alarm_control_panel:

• platform: mqtt
  state_topic: homeassistant/alarm
  command_topic: homeassistant/alarm/set
  payload_disarm: Disarm
  payload_arm_home: ArmHome
  payload_arm_away: ArmAway
  name: Alarme Visonic

in the configuration.yaml, it need to be placed under the mqtt.yaml:
alarm_control_panel:
- name: Alarme Visonic
unique_id: alarme_visonic
state_topic: homeassistant/alarm
command_topic: homeassistant/alarm/set
payload_disarm: Disarm
payload_arm_home: ArmHome
payload_arm_away: ArmAway