Add information, where to find custom boards

[rubo77]

It would be cool to have a playce where people can post custom boards, they like and link that list in the Readme.

Maybe use the Wiki here: https://github.com/smack42/DriftingDroids/wiki/

The site should be ordered by categories:

• standard boards
• custom board-size
• custom walls

This site should also be linked in the dialog to load a new custom game

[smack42]

I don't want to host such a service myself.

At first, one would have to promote the service to attract users to contribute to it and then, once it's popular, it will need permanent moderation to keep the negative elements (like trolls and spammers) in check.

[rubo77]

If users could vote on the boards, this would regulate itself then