Add root vs. non-root to set up guide.

docs/how-to/set-up-backups.md

@@ -27,6 +27,13 @@ export PATH="$PATH:~/.local/bin"
 Note that your pip binary may have a different name than "pip3". Make sure
 you're using Python 3, as borgmatic does not support Python 2.
 
+### root versus non-root
+
+These instructions install and run borgmatic as root, because many users want
+to backup system files that require privileged access. Therefore borgmatic
+needs to run as root to read them. But if you don't need to backup such files,
+then you are welcome to install and run borgmatic as a non-root user.
+
 ### Other ways to install
 
 Along with the above process, you have several other options for installing