Add a Setup Guide for iOS Wi-Fi IPv6 DNS

When connected to a Wi-Fi network on iOS you can manually edit the IPv6 DNS (if available)

1. Go to iOS setting
2. Go to "Wi-Fi"
3. Next to the connected Wi-Fi tap the "i" icon
4. Scroll down to "Configure DNS"
5. Select "Manual"
6. Enter the NextDNS IPv6 addresses there