doc: Document auditing facilities

refs #2584

doc/15-Auditing.md

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+# Auditing <a id="auditing"></a>
+
+Auditing in Icinga Web 2 is possible with a separate [module](https://github.com/Icinga/icingaweb2-module-audit).
+
+This module provides different logging facilities to store/record activities by Icinga Web 2 users.
+
+Icinga Web 2 currently emits the following activities:
+
+## Authentication
+
+Activity | Additional Data
+---------|----------------
+login    | username
+logout   | username