Document how to install fish quitcd function

It was not clear that the name of the file should match
the name of the function.

scripts/quitcd/quitcd.fish

@@ -1,3 +1,6 @@
+# Rename this file to match the name of the function
+# e.g. ~/.config/fish/functions/n.fish
+
 export NNN_TMPFILE="/tmp/nnn"
 
 function n --description 'support nnn quit and change directory'