Add docs for rattail backup command

docs/backup/running.rst

@@ -0,0 +1,46 @@
+
+.. highlight:: sh
+
+=====================
+ Running the Command
+=====================
+
+If you have installed and configured the backup app, you can run it
+like so::
+
+   cd /srv/envs/backup
+   sudo -H bin/rattail -c app/rattail.conf backup
+
+Overnight tasks should probably use ``app/silent.conf`` instead::
+
+   cd /srv/envs/backup
+   sudo -H bin/rattail -c app/silent.conf backup
+
+Either of those will cause Rattail to invoke its backup logic.  Since
+neither of those passes any extra args to the backup command, the
+behavior will be determined only by the config file.
+
+If you are following along in order, and only have the "sample"
+``rattail.conf`` file in place, that tells Rattail to:
+
+  * dump all databases to file (for postgres and mysql)
+  * do *not* run ``rsync``
+  * do *not* run ``borg create``
+
+But of course that is just for the sample; your config file may tell
+Rattail to do something else.
+
+
+Command Parameters
+==================
+
+The above shows "normal" use, when you just run ``rattail backup`` and
+do not specify any params.  But there are some available, e.g. so you
+can run just the ``rsync`` part but nothing else, despite what config
+says.
+
+They are not yet documented here; for now please see the built-in
+help::
+
+   cd /srv/envs/backup
+   sudo -H bin/rattail -c app/rattail.conf backup --help