Add something explaining the game rules

[bryanbraun]

Some content for the rules already exists in the "description" meta tag.

Ideas:

• Make the rules really succinct and just put them somewhere on the page.
• It could be a link next to the "source code" link, that opens the rules in a separate modal or popup window.
• Add a Details / Summary accordion for a really simple implementation. Maybe it could go below the playing grid and have a + icon next to it to show that it can be expanded/collapsed?