Add reference to the "Introducing OData" white paper

[Lingxi-Li]

As a completely new guy to the OData technology and even REST, I find this white paper quite helpful, despite being somewhat out of date. It would be nice if there could be a reference or mention to this well-written introductory material on the home page of odata.org. Currently, the website lacks such a systematic, well organized, yet simple introduction to OData for the absolutely new. This white paper is a nice fill to the gap.