Add inline docs to mousePressed listener

[j6k4m8]

@lmccart Added docs! Hopefully now it's more clear what's happening here.

[j6k4m8]

By the way, I did not commit the build files (lib/) when I pushed; do you push those by convention, or do you have a central build/deploy process?

[lmccart]

@j6k4m8 there's a build process. the lib/p5.* files should not be included, and are .gitignored
https://github.com/processing/p5.js/blob/master/.gitignore#L6

[lmccart]

comments are great, thanks!