Document adding alert rules

[moan0s]

This is a PR as using this technique requires manual intervention on the server and we can question if wen want that to be an "offical" way of doing things. As I benefit from doing things like that I'd argue yes but I'm open to discussion

[spantaleev]

This example could also make use of ansible-role-aux role and manage the /mash/prometheus/config/alert.yml file using a aux_file_definitions variable.

Example:

aux_file_definitions:
  - dest: "{{ prometheus_config_path }}/alert.yml"
    owner: "{{ prometheus_uid }}"
    group: "{{ prometheus_gid }}"
    content: |
      something
      another