Amend Readme regarding /etc-dnsmasq.d

Signed-off-by: yubiuser <github@yubiuser.dev>

README.md

@@ -76,7 +76,7 @@ services:
     volumes:
       # For persisting Pi-hole's databases and common configuration file
       - './etc-pihole:/etc/pihole'
-      # Uncomment the below if you have custom dnsmasq config files that you want to persist. Not needed for most.
+      # Uncomment the below if you have custom dnsmasq config files that you want to persist. Not needed for most starting fresh with Pi-hole v6. If you're upgrading from v5 you and used it before your should keep it enabled at the first v6 container start for a complete migration. Can be turned off afterwards.
       #- './etc-dnsmasq.d:/etc/dnsmasq.d'
     cap_add:
       # See https://github.com/pi-hole/docker-pi-hole#note-on-capabilities