Add "Adding custom domain" section to README.md

[ByPort]

Users asks to add the ability to add custom domains. Until this feature is implemented, custom domains can be hardcoded. This PR adds a quick guide to the README.md on how to add custom domains.

Based on this comment.

[plutonium-239]

Check the original thread https://github.com/manojVivek/medium-unlimited/issues/105 I've explained there how to add it. Yes, it would be easier if there was just an option for it with some GUI but this isn't hard either.

[Aarekaz]

Yeah, I tested it and it works. Do add it in the pullrequest. It would be very easy to find afterwards.

[ByPort]

Added it, thanks for notifying