mention --save-space at the place we talk about keeping disk space free

docs/quickstart.rst

@@ -17,7 +17,8 @@ a good amount of free space on the filesystem that has your backup repository
 
 If you run out of disk space, it can be hard or impossible to free space,
 because |project_name| needs free space to operate - even to delete backup
-archives.
+archives. There is a `--save-space` option for some commands, but even with
+that |project_name| will need free space to operate.
 
 You can use some monitoring process or just include the free space information
 in your backup log files (you check them regularly anyway, right?).