documentation: FAQ: added item "How can I backup from non-btrfs hosts?"

doc/FAQ.md

@@ -1,6 +1,34 @@
 btrbk FAQ
 =========
 
+How can I backup from non-btrfs hosts?
+--------------------------------------
+
+First create a btrfs subvolume on the backup server:
+
+    # btrfs subvolume create /mnt/btr_backup/myhost_sync
+
+In your daily cron script, prior to running btrbk, sync your source to
+`myhost_sync`, something like:
+
+    rsync -a --delete -e ssh myhost.mydomain.com://data/ /mnt/btr_backup/myhost_sync/
+
+Then run btrbk, with myhost_sync configured *without any targets* as
+follows:
+
+    volume /mnt/btr_backup
+      subvolume myhost_sync
+        snapshot_name    myhost
+
+        snapshot_create  always
+        snapshot_preserve_daily    14
+        snapshot_preserve_weekly   20
+        snapshot_preserve_monthly  all
+
+This will produce daily snapshots `/mnt/btr_backup/myhost.20150101`,
+with retention as defined with the snapshot_preserve_* options.
+
+
 Why is it not possible to backup '/' (btrfs root) ?
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