doc(tfstated): import documentation file from the forgejo wiki

doc/Backup.md

@@ -0,0 +1,39 @@
+# Backup
+
+All TfStated data is stored in its SQLite database. Backing up this database can
+be achieved through different means.
+
+## VACUUM INTO
+
+SQLite has a `VACUUM` command that is used to optimize the structure of the
+database. It also has the side effect of shrinking its size by reclaiming disk
+space that is no longer used by table data.
+
+`VACUUM INTO` is a variant that runs this process without interfering with other
+writes and also outputs a perfect working copy of your database at the point in
+time you run this command. Everything is kept as is, including the `PRAGMA` and
+indexes. You can then backup this output file. Example:
+
+``` sql
+umask 077
+echo -n "VACUUM INTO '/tmp/tfstated.db';" | sqlite3 /var/lib/tfstated/tfstated.db
+```
+
+This is the recommended way to backup your TfStated database as you can pair it
+up with the backup software of your choice (for example
+[borg](./Example-Backup-With-Borg)).
+
+Another option is to output this backup file
+directly to a remote storage mounted on your server.
+
+## Litestream
+
+[Litestream](https://litestream.io/) is a background service that continuously
+monitors and replicates an SQLite database to an S3 compatible object storage.
+
+Litestream is a good backup solution that has the advantage of providing point
+in time recovery. The downsides are that it is another service to manage and
+that it requires a write lock on the database when its checkpointing occurs.
+
+Since Litestream is more complex to operate, the TfStated project recommends to
+pair this solution with a `VACUUM INTO` style backup.