[cddlm] updated document

[Steve Loughran]

This is the updated document. It is not yet ready to submit, because one 
of the spelling changes affects the API schema.

In the document I have moved from URL schema to URL scheme, which is the 
correct terminology for a scheme such as http:. However there is a 
<schema> element in the addFile request. We need to change this, sooner 
rather than later, and once the change is made, also update the relevant 
bits of the document, including the appendix xsd and the embedded example.

1. Whose code is going to break when I change the name of 
AddFileRequest/schema to AddFileRequest/scheme

2. Do we need to increment the API XSD numbering in the process?

-steve