Added coverage status button to README

This is a follow-up to #665 and #678, adding the coveralls.io status button to the `readme.md`. As @suralc pointed out, it might be advisable to wait a bit until coverage has been increased before merging this pr in :wink: 

For reference, the current status is this: [![Coverage Status](https://coveralls.io/repos/yiisoft/yii2/badge.png?branch=master)](https://coveralls.io/r/yiisoft/yii2?branch=master)