docs: Add migrate docs

docs/chapters/migration.rst

@@ -1,7 +1,44 @@
-=========
 Migration
 =========
 
+Bastille
+--------
+
+Bastille supports migrations to a remote system using the ``migrate`` subcommand.
+
+Prerequisites
+^^^^^^^^^^^^^
+
+There are a couple of things that need to be in place before running the ``migrate`` command.
+
+First, you must have bastille configured both locally and remotely to use the same filesystem
+configuration. ZFS on both, or UFS on both.
+
+Second, we need a user on the remote system that has key-based authentication set up. Bastille
+does not support password-based authentication. This also implies that SSH is working on the
+remote system. This user also needs ``sudo`` permissions on the remote system.
+
+Third, if updating from a previous version, please make sure to ``diff`` your ``bastille.conf``
+file, as the ``migrate`` sub-command adds a new variable called ``bastille_migratedir``.
+
+Migration
+^^^^^^^^^
+
+To migrate a jail (or multiple jails) we can simply run
+``bastille migrate TARGET USER HOST``. This will export the jail(s), send them to the
+remote system, and import them.
+
+The ``migrate`` sub-command includes the ``-a|--auto`` option, which will auto-stop the old jail,
+migrate it, and attempt to start the migrated jail on the remote system after importing it. See the
+warning below about auto-starting the migrated jail.
+
+WARNING: Every system is unique, has different interfaces, bridges, and network configurations.
+It is possible, with the right configuration, for jails to start and work normally. But for some
+systems, it will be necessary to edit the ``jail.conf`` file of the migrated jail to get it working
+properly.
+
+You can optionally set ``-d|--destroy`` to have Bastille destroy the old jail on completion.
+
 iocage
 ------
 

docs/chapters/subcommands/migrate.rst

@@ -0,0 +1,18 @@
+migrate
+=======
+
+The ``migrate`` sub-command allows migrating the  targeted jail(s) to
+another remote system. See the chapter on Migration.
+
+This sub-command supports multiple targets.
+
+.. code-block:: shell
+
+  ishmael ~ # bastille migrate help
+  Usage: bastille migrate [option(s)] TARGET USER HOST
+
+    Options:
+
+    -a | --auto             Auto mode. Start/stop jail(s) if required.
+    -d | --destroy          Destroy local jail after migration.
+    -x | --debug            Enable debug mode.