documentation: add a tip

README.md

@@ -122,3 +122,18 @@ and create a symbolic link, this will allow a drop-in replacement
 without rebooting.
 
 Then, update your configuration file to add the new entry.
+
+## How does a good configuration file look
+
+There are no good or bad configuration file content (except if it's
+invalid obviously). The whole point of impermanence is to hand-pick
+every directories and files you want to run your session, by admitting
+all others files will be thrown away at reboot.
+
+While you can list `.config` and `.local` which is a very large
+include, you could rather list only a subset of those, which will
+make a long list and require a few guess&fix sessions to get the
+things right.
+
+The less the directories are at the top level, the more you will
+pinpoint the exact configuration you want to keep over time.