Update README.md
custom pseudo-element work

BUG=

Review URL: https://chromiumcodereview.appspot.com//14312011
Version 0.4.8+2

BUG=

Review URL: https://chromiumcodereview.appspot.com//15747002
Use new csslib supporting nested rules and pseudo-elements

BUG=

Review URL: https://chromiumcodereview.appspot.com//15738008
v0.4.8+3

BUG=

Review URL: https://chromiumcodereview.appspot.com//15744008