quickstart: add note about permissions, borg@localhost, fixes #3452

docs/quickstart.rst

@@ -55,6 +55,33 @@ Also helpful:
     - Other tasks fill the disk simultaneously
     - Hard quotas (which may not be reflected in statvfs(2))
 
+Important note about permissions
+--------------------------------
+
+Using root likely will be required if you want to backup files of other users
+or the operating system. If you only back up your own files, you neither need
+nor want to use root.
+
+Avoid to create a mixup of users and permissions in your repository (or cache).
+
+This can easily happen if you run borg using different user accounts (e.g. your
+non-privileged user and root) while accessing the same repo.
+
+Of course, a non-root user will have no permission to work with the files
+created by root (or another user) and borg operations will just fail with
+`Permission denied`.
+
+The easy way to avoid this is to always access the repo as the same user:
+
+For a local repository just always invoke borg as same user.
+
+For a remote repository: always use e.g. borg@remote_host. You can use this
+from different local users, the remote user accessing the repo will always be
+borg.
+
+If you need to access a local repository from different users, you can use the
+same method by using ssh to borg@localhost.
+
 Automating backups
 ------------------