note performance and chrome-support issues.

js/pixl/README.md

* multiplayer backend as for pixl
* live-editing from a built-in editor (similar to firefox' scratchpad,
  but on the page. should also support saving code to `localStorage`.)
* better performance. noticable lag at 10000 trixls, probably
  batch-drawing would help, but how? first find out what the bottleneck
  is.
* bug: sometimes when moving the mouse to certain positions (edges of
  the screen?) the camera rapidly alternates between two positions
* bug: doesn't work in chrome, as it's missing support for iterators and
  the `keys()` function on maps. i didn't find any convenient polyfills
  (`keys()` should return something iterator-like, e.g. returning raw
  values on `next()` and `StopIteration` when the end is reached.)