Improve README/docs to clarify intended use cases

[LukasKalbertodt]

This is very much not intended to generate HTML. There are also many use cases where a serialization library is more fitting. But there are still cases where one needs to generate fairly fixed XML where this library could be useful. Generating RSS and SVG code are two specific examples I can come up with.

Maybe the README and some docs shouldn't use a complete dummy example, but one of the above ones instead.