Add docs for configuring white label URL

[AlexVipond]

Which DNS records need to be added to make sure open and link tracking works with a white label URL? And which records allow the user to open the app through a nicer URL (e.g. website subdomain)?

[goehlemichael]

For example:
You can create an A record such as "email.customdomain.com" and have it point to the ip of your sever. You can then use that url as the whitelist label