schema: add README, as requested by Noel

Change-Id: If3270ad1d20002dc39a5b4326b0cb42753112e93
Reviewed-on: https://gerrit.libreoffice.org/56868Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
Tested-by: Michael Stahl <Michael.Stahl@cib.de>

schema/README

+schemas that can be used for validating ODF files
+
+subdirs:
+- mathml2: W3C MathML 2.0 XML Schema (needed for Math embedded objects)
+- odf1.0, odf1.1, odf1.2: official OASIS RelaxNG schemas
+- odf1.3: current OASIS draft ODF 1.3 RelaxNG schema
+- liberoffice: draft ODF schema, with additional LO extensions
+
+The extension schema in "libreoffice/" is used by all unit tests if
+--with-export-validation is given, which is the default.
+
+Notably this means that if you add a new feature to the ODF filters and you add
+the required unit test for the new feature, then most likely the test will fail
+with a complaint from the validator; in this case the schema needs to be
+updated to contain the new elements and attributes.
+
+The extension schema uses the RelaxNG "include" feature to refer to the ODF
+schema; this means that it only contains those parts of the schema that
+actually need to be changed - this works well in many cases because the ODF
+schema is quite well structured with many named patterns, but unfortunately
+there are a few places where that isn't the case and large chunks needed to be
+copied to override them.
+
+In the easy case, to add an attribute you just want to search for the
+corresponding element, which will have a "foo-attlist" named pattern, and then
+add another attribute like this:
+
+  <rng:define name="draw-enhanced-geometry-attlist" combine="interleave">
+    <rng:optional>
+      <rng:attribute name="drawooo:sub-view-size">
+        <rng:ref name="string"/>
+      </rng:attribute>
+    </rng:optional>
+  </rng:define>
+
+Currently only the features that are actually exported in the unit tests have
+been added to the schema; there is still some work to do here to add
+everything; the crashtesting script also does ODF validation of all files and
+now also uses the custom schema.
+
+Unfortunately it turned out that there are a lot of extensions already for
+which no proposal exists [1], and in many cases not even an entry on the Wiki
+[2], so clearly something like this extension schema is needed.
+
+[1] git grep TODO schema/libreoffice
+[2] https://wiki.documentfoundation.org/Development/ODF_Implementer_Notes/List_of_LibreOffice_ODF_Extensions
+