Add backup/restore stuff

Somehow got lost, probably my mistake.

docs/backup-restore.md

@@ -2,4 +2,58 @@
 title: Back-up and restore an application
 ---
 
-TODO.
+We'll use the example of a [`coop-cloud/wordpress`][wordpress] app deployed at
+`blog.example.com`.
+
+## Backing up
+
+```
+abra app wordpress_blog_example_com backup --all
+```
+
+This will download backups of the Wordpress files (plugins, themes, and uploads)
+and database (posts, settings and users) to the default backup directory,
+`~/.abra/backups`.
+
+You can also back up just the files:
+
+```
+abra app wordpress_blog_example_com backup app
+```
+
+or just the database:
+
+```
+abra app wordpress_blog_example_com backup db
+```
+
+!!! warnings
+Not all types of apps know how to do backups yet -- if you see a message like
+`ERROR: 'nextcloud' doesn't know how to do app backups`, then extra code is
+needed in that app's `abra.sh` -- you might be able to add this yourself based
+on [`coop-cloud/wordpress` `abra.sh`][wordpress_abra.sh], otherwise please
+open a new issue (in this case for [`coop-cloud/nextcloud`][nextcloud]).
+
+## Restore
+
+You can restore data into a running application. This is useful to restore an
+app to a previous state, to migrate an app from one Co-op Cloud server to
+another, or to help move an app to Co-op Cloud initially.
+
+Using the same example app above, you can restore files:
+
+```
+abra app wordpress_blog_example_com restore blog_example_com_app.tar.gz
+```
+
+and/or the database:
+
+```
+abra app wordpress_blog_example_com restore blog_example_com_db.sql.gz
+```
+
+(there isn't yet a command to restore files and database data at the same time)
+
+[wordpress]: https://git.autonomic.zone/coop-cloud/wordpress
+[wordpress_abra.sh]: https://git.autonomic.zone/coop-cloud/wordpress/src/branch/master/abra.sh
+[nextcloud]: https://git.autonomic.zone/coop-cloud/nextcloud