Update documentation explaining the source package

debian/kali-themes.preinst

@@ -15,11 +15,20 @@ FILES_TO_DIVERT="
 /etc/xdg/xfce4/xfconf/xfce-perchannel-xml/xsettings.xml
 " # END FILES_TO_DIVERT
 
+setup_diversion_on_upgrade() {
+    local prev_version="$1"
+    local ref_version="$2"
+    local file="$3"
+    if dpkg --compare-versions "$prev_version" lt "$ref_version"; then
+	setup_diversion "$file"
+    fi
+}
+
 setup_diversion() {
-    local file=$1
+    local file="$1"
     dpkg-divert --rename --package kali-themes \
-		--divert $file.original \
-		--add $file
+		--divert "$file.original" \
+		--add "$file"
 }
 
 case "$1" in
@@ -39,6 +48,9 @@ case "$1" in
     ;;
     upgrade)
 	# Dynamically add new diversions when required on upgrade
+	# Example to divert /etc/xdg/xfce4/panel/default.xml when you
+	# upgrade from a version older than 2019.4.6:
+	# setup_diversion_on_upgrade $2 2019.4.6 /etc/xdg/xfce4/panel/default.xml
     ;;
 esac