Adds active_directory to the autoinstall reference

documentation/autoinstall-reference.md

@@ -406,6 +406,24 @@ The hostname for the system.
 
 The password for the new user, crypted. This is required for use with sudo, even if SSH access is configured.
 
+### active_directory
+
+**type:** mapping, see below
+**default:** no default
+**can be interactive:** yes
+
+Accepts data required to join the target system in an Active Directory domain.
+
+A mapping that can contain keys, all of which take string values:
+
+#### admin_name
+
+A domain account name with privilege to perform the join operation. That account's password will be requested during runtime.
+
+#### domain_name
+
+The Active Directory domain to join.
+
 ### ubuntu-pro
 
 **type:** mapping, see below