qfeatures.txt: add section about XML schema

... so the XML schema part can be disabled in the xmlpatterns module,
while still using other functionality (e.g. XPath).

Change-Id: I3599ea8f915d34e0b1ba5d6a30f3f6269a4c992e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>