add comment to SnesInput

main.js

@@ -12,6 +12,9 @@ function bound(low, x, high) {
   return Math.max(low, Math.min(x, high));
 }
 
+// Representation of the state of the buttons on an SNES controller. (This may
+// be the result of keyboard or gamepad input, which get mapped to the
+// standard SNES buttons.)
 class SnesInput {
   constructor() {
     this.up = false;