short note about trixl in readme.

js/pixl/README.md

it's live at <http://pixl.papill0n.org>.
[whirlwind intro]: https://github.com/heyLu/codegirls/blob/master/2013-12-17-christmas-special.md
# trixl!
`pixl` in 3 dimensions. you can even import the data from pixl.
see it live at <http://pixl.papill0n.org/3>.