Add system.autoupdates state (#61)

* Add support for autoupdates

only Debian-based systems are supported for now
(through unattended-upgrades package)

* Fix test on system.autoupdates.pkgs
diff --git a/README.rst b/README.rst
index c96ac59..cf61802 100644
--- a/README.rst
+++ b/README.rst
@@ -91,6 +91,21 @@
             repo: 'custom-repo'
             verify: false
 
+Linux with autoupdates (automatically install security package updates)
+
+.. code-block:: yaml
+
+    linux:
+      system:
+        ...
+        autoupdates:
+          enabled: true
+          mail: root@localhost
+          mail_only_on_error: true
+          remove_unused_dependencies: false
+          automatic_reboot: true
+          automatic_reboot_time: "02:00"
+
 Linux with cron jobs
 
 .. code-block:: yaml