Add rng schema for model.xml

Change-Id: I1b75c5c42a131c7994868ea3261120c6a5b7650e

writerfilter/documentation/ooxml/model.xml

-These are various notes about ooxml/model.xml and related stuff. They have been
-mostly found out by trial and error, because existing documentation is poor
-or nonexistent, so I don't actually understand writerfilter that much (and
-think nothing nice about it) and don't think it (both writerfilter and my
-understanding/liking of it) could be noticeably improved. In an ideal world
-it should be nuked from orbit and started again from scratch with a saner design.
-
--
-CT_xxx (Complex Type) - it seems to be used for XML elements
-ST_xxx (Simple Type) - it seems to be used for XML attributes
-
-- SPRM (the Sprm structure specified a modification to a property of a
-character, paragraph, table, or section in the binary .doc format) - in
-the context of OOXML it seems to pretty much mean "XML element"
-
--
-
-Format of the <resource> tag (shortened CT_Font example):
-
-    <resource name="CT_Font" resource="Properties" tag="font">
-      <element name="charset" tokenid="ooxml:CT_Font_charset"/>
-      <attribute name="name" tokenid="ooxml:CT_Font_name"/>
-    </resource>
-
-CT_Font is the type that is defined how it will be handled.
-resource="XXX" means it will be handled by OOXMLFastContextHandlerXXX class
-no idea what tag="font" means or if it matters
-<element> defines the <w:charset> subelement will be handled in sprm() function
-    as NS_ooxml::LN_CT_Font_charset case
-<attribute> defines the <w:name> attribute of the element will be handled
-    in attribute() function as NS_ooxml::LN_CT_Font_name case
-in both cases sprm()/attribute() may mean actually any of the various strange
-    naming ideas like lcl_sprm()
-
--
-If an element (and its subelements) are not processed but the element itself
-does not require any special handling, make sure something like the below is present.
-Otherwise null context will be created and the element and all its subelements
-will be ignored.
-
-<resource name="CT_OMathPara" resource="Stream" tag="math"/>
-