README.md: add a usage note to Packaging

Packaging is only interesting to maintainers and packagers, not end
users. So I think it's a good idea to put a small warning/note at the
top of the packaging section to keep users from falling down rabbit
holes.

README.md

@@ -24,6 +24,9 @@ There are currently two options for installing `zpm`.
 
 ## Packaging
 
+Note: This section is interesting only for project maintainers and packagers.
+This is not required for installing and using `zpm`.
+
 1. Install debian packaging dependencies:
 
     $ sudo apt-get install devscripts debhelper