As playing the game is supposed to be intuitive, instructions are not included. The cheat levels are not intuitive, but if you plan to use them, you should make the effort to figure out what they do! I welcome feedback about the game, and its ease of use.
There's a bug in some browsers that may cause a game to be restarted if the browser window is resized. So beware!
Descriptions of the game of Mastermind are widely available. For example, see Ben's Marvelous Mastermind Game).
Here's the (yet undocumented) source for the game.