add doc
# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Your branch is up-to-date with 'origin/master'.
#
# Changes to be committed:
#	new file:   LICENSE
#	modified:   README.md
#	new file:   img/screenshot.png
#	new file:   js/index.js
#	new file:   js/jquery-1.7.1.min.js
#	new file:   js/jquery-ui-1.8.18.custom.min.js
#	new file:   js/message.js
#	new file:   js/sh.drawable.js
#	new file:   js/sh.js
#	new file:   js/sh.layer.js
#	new file:   js/sh.settings.js
#	new file:   js/sh.stage.js
#