Change 'require' to conditional 'javascript_include_tag' in readme

The readme recommends adding to application.js:
```ruby
//= require selectivizr
```

Problem: This loads selectivizr for ALL browsers.
 
Selectivizr [recommends](http://selectivizr.com/) adding a HTML conditional for browser version, so I've changed the readme to reflect this.

Selectivizr also requires including the JS library (JQuery, Prototype, etc.) prior to the selectivizr include, so I added that to the readme as well.