RDDL: minor update

I've placed a minor and interim update to the RDDL spec 
at http://www.rddl.org/ -- it contains fixes to some 
errata, some rewording, and inclusion of a RELAXNG 1.0 
schema for RDDL.

No substantial changes.

More work to follow -- let me know if your favorite 
erratum hasn't yet made the fix list.
--
Jonathan