Add notifications (e.g. pagerduty) to 'addons & plugins' docs

fixes #6714

doc/13-addons-plugins.md

@@ -84,6 +84,25 @@ is even simpler these days.
 
 More details can be found in [this blog post](https://www.icinga.org/2014/12/02/team-icinga-at-osmc-2014/).
 
+## <a id="notification-scripts-interfaces"></a> Notification Scripts and Interfaces
+
+There's a variety of resources available, be it different notification scripts such as:
+
+* E-Mail ([examples](3-monitoring-basics.md#notifications) provided)
+* SMS
+* Pager (XMPP, etc.)
+* Twitter
+* IRC
+* Ticket systems
+* etc.
+
+Additionally external services can be integrated with Icinga 2:
+
+* [Pagerduty](https://www.pagerduty.com/docs/guides/icinga2-integration-guide/)
+
+More information can be found on the [Icinga Website](https://www.icinga.org)
+and the [Icinga Wiki](https://wiki.icinga.org/display/howtos/Home).
+
 ## <a id="configuration-tools"></a> Configuration Management Tools
 
 If you require your favourite configuration tool to export Icinga 2 configuration, please get in

doc/3-monitoring-basics.md

@@ -917,6 +917,8 @@ refining the notifications being actually sent.
 There are many ways of sending notifications, e.g. by e-mail, XMPP,
 IRC, Twitter, etc. On its own Icinga 2 does not know how to send notifications.
 Instead it relies on external mechanisms such as shell scripts to notify users.
+More notification methods are listed in the [addons and plugins](13-addons-plugins.md#notification-scripts-interfaces)
+chapter.
 
 A notification specification requires one or more users (and/or user groups)
 who will be notified in case of problems. These users must have all custom