Add instructions on updating from moedict-webkit

[audreyt]

I've just retroactively shrunk the moedict-webkit repo by moving all Cordova-specific bits into this one. Although this repo is now stand-alone, its README should reflect on how to upgrade the www/ files from an moedict-webkit checkout, ideally with this repo checked out as a sub-directory under moedict-webkit/moedict-app/.

Also @a-tsioh and @ethantw : Please git pull on your active moedict-webkit branches (basic_zhuyin and user-pref respectively) to get the new baseline history before committing & pushing new work. Thank you!