Document how to deserialize and serialize XML with namespaces using serde

[eirnym]

There's a few examples here and there, I'd like to gather them to documentation. There's 2 cases with using namespaces used differently: When all namespaces are defined as attributes of a root element of the document, and namespaces defined inside child element.

Maybe it would be obvious to define them, but it would be better, when these examples would be documented properly, including xsi:... attributes

[Mingun]

Namespaces with serde does not officially supported yet, the progress is tracked in #218. Some limited support exists currently, but you are limited to use concrete prefixes, which definitely will not work in all situations.

I'm open to contributions to resolve #218 and on suggestions to better document existing behavior.

[eirnym]

Even documentation of the current state will be enough for now. And I really mean for both Serialization AND Deserialization