Add RELAX NG schema for validation.

Hi! Here's the pull request as we discussed. You might want to have a look at the "disclaimer" I added to the text at index.html, do you think that's good?

Also, there's a few questions in the schema source regarding permitted "type" attribute values, I have only allowed the ones used in your example. Better to be strict at first and then add things as they are needed?

git commit note:
Checking in appdata.rng even though it is generated from
appdata.rnc using trang and the supplied Makefile, for
simplicity.