title: Sending a common 'text' with all Prometheus alert emails
body: <p>I have installed one monitoring setup in Kubernetes. I have used Promethuese, and for alerting I have configured alert manager. Everything is working fine.</p>
  <p>I have one requirement to send a common text, say for eg:- a wiki URL with all the alert emails. I can do it by editing all the alert blocks, but is there a way to add just this line in a commonplace so that it will be picked up whenever alert manager sends an email?</p>
  <pre><code>alertmanagerFiles:
    alertmanager.yml:
      global:
        resolve_timeout: 5m
        smtp_smarthost: 'xxxxx:portnumber'
        smtp_from: 'email@domain.com'
        smtp_auth_username: 'xxxx'
        smtp_auth_password: 'xxxxx'
        smtp_require_tls: false
      route:
        group_by: ['alertname', 'cluster', 'service']
        group_wait: 30s
        group_interval: 5m
        repeat_interval: 30m
        receiver: ops
      inhibit_rules:
      - source_match:
          severity: 'critical'
        target_match:
          severity: 'warning'
        equal: ['alertname', 'cluster', 'service']
      receivers:
        - name: 'ops'
          email_configs:
          - to: 'xxxxx'
            send_resolved: true
          - to: 'xxxxx'
            send_resolved: true
  </code></pre>
  <p>This is the alert manager config</p>