Sect 7.3 - remove contraints

From Yadis

In section 7.3 it would be helpful to differentiate between "Yadis constraints on the schemas", and where we are re-stating the schemas as they stand. Two constraints worth removing:

7.3.3, "MUST contain one or more Service elements." This is not a necessary constraint. True, a document with no Service elements is of limited use to an application, but it needn't be an invalid case. An example may be someone with a hosted Yadis service with no services yet configured. An application will probably treat this much the same as a document that contained Services only of types it was not interested in. Instead of focusing on the minimum requirement here, it might be more productive to say what may be included. e.g. "may contain as many Service elements as necessary to describe the Yadis services."

7.3.4 "MUST contain at least one Type element" There is a need for people to know "How to define a new Yadis Service." In answering that, we can state that Yadis Service designers should define a Type value and require its presence in the Service element for use with their protocol. "Type" is, however, not a required element in the XRD schema. I do not believe a Yadis parser should treat a Service element with no Type as an error, as they might do after reading the MUST here.