StackOverflow policy

@harvesthq/chosen-developers I got this idea while browsing the [Poltergeist repo](https://github.com/jonleighton/poltergeist/tree/v1.3.0#getting-help). It makes more sense to ask people to use tags when asking questions on StackOverflow. There are already tags created for chosen

* [chosen.js](http://stackoverflow.com/questions/tagged/chosen.js)
* [jquery-chosen](http://stackoverflow.com/questions/tagged/jquery-chosen)
* [prototype-chosen](http://stackoverflow.com/questions/tagged/prototype-chosen)

I kind of like the idea of moving this to a Getting Help section in the README, like it is on the Poltergeist repo. I doubt if people creating Issues ever dig into the Contributing page. Thoughts?

Here's the [contributing.md](https://github.com/harvesthq/chosen/blob/stackoverflow_policy/contributing.md) file for review: