vuejs/vuejs.org — pull request #28 by BigBlueHat (2014-07-09 16:39:38+00:00)
Title: added note about $data and mutation object

Found these useful things in:
https://github.com/yyx990803/vue/issues/328#issuecomment-46985518
and
https://github.com/yyx990803/vue/issues/224#issuecomment-39796607