HDP: Revised Introductory Material

I added Abstract and Status of the Document sections.

I also rewrote the Introduction section, mainly to clarify the  
distinction between document conformance requirements (the conforming  
language) and implementation requirements (the supported language). As  
I revise the individual principles in response to feedback, I will try  
to make clear how they relate to the dual nature of the spec.

You can see the results at <http://dev.w3.org/cvsweb/~checkout~/html5/html-design-principles/Overview.html?content-type=text/html;%20charset=iso-8859-1 
 >.

Regards,
Maciej