Updated README to include Twitter Bootstrap in Credits

The reason this relatively small change is being added to the master branch as a `Pull Request` is because it marks the decision to use [Twitter Bootstrap](http://getbootstrap.com) as the framework for the front end, ensuring that in future releases, we'll be able to optimize [OSERP](https://github.com/bengrunfeld/open-source-erp) for mobile devices.

Initially, all automatic re-styling for mobile devices and smaller screen sizes will be turned off and the app will operate at a fixed width of `1170px`.