README: note about AppArmor

README.md

@@ -31,6 +31,13 @@ then modify `/etc/cups/cups-pdf.conf`:
 PostProcessing /path/to/cups-pdf-postprocess-your-wrapper.sh
 ```
 
+*NOTE:* Ubuntu requires modifying AppArmor settings to allow cups-pdf to execute
+this script by adding something like
+```
+  /path/to/cups-pdf-postprocess-your-wrapper.sh ux,
+```
+into cups-pdf profile.
+
 *WARNING:* The script does not delete printed files, so files printed through cups-pdf
 will stay where they were printed to (default path is `/var/spool/cups-pdf/${USER}`).
 It is up to you to delete them when needed. Easy way to do so might be to move