Add in-game instruction

[Hyperseeker]

The game has instructions posted in this repository, but none within itself. New players may find themselves baffled by a game with no explanation.

Some form of lightweight guidance could come in handy for new and returning players alike. Perhaps storing the boolean for "player knows how to play / seen tutorial" in localStorage would come in handy if the gameplay doesn't change much.

One potential solution is to softly promote the right action (tapping in mobile mode, pressing the key on the keyboard) via visual effects (animating the "pressed" state in and out and placing a keyboard next to a random key, respectively).