xmloff: more README

Change-Id: I951c5db621be2443961f5d2a1a0ca6a2684f8e0f

xmloff/README

-Contains common xml import and export filter logic.
+Contains ODF import and export filter logic.
+
+The main library "xo" contains the basic ODF import/export filter
+implementation for most applications.  The document is accessed
+via its UNO API, which has the advantage that the same import/export
+code can be used for text in all applications (from/to Writer/EditEngine).
+The filter consumes/produces via SAX UNO API interface (implemented in
+"sax").  Various bits of the ODF filters are also implemented in
+applications, for example sw/source/filter/xml.
+
+There is a central list of all element or attribute names in
+xmloff/inc/xmloff/token.hxx.  The main class of the import filter
+is SvXMLImport, and of the export filter SvXMLExport.
+
+The Import filter maintains a stack of contexts for each element being
+read.  There are many classes specific to particular elements, derived
+from SvXMLImportContext.
+
+Note that for export several different versions of ODF are supported,
+with the default being the latest ODF version with "extensions", which
+means it may contain elements and attributes that are only in drafts of
+the specification or are not yet submitted for specification.  Documents
+produced in the other (non-extended) ODF modes are supposed to be
+strictly conforming to the respecive specification, i.e., only markup
+defined by the ODF specification is allowed.
+
+There is another library "xof" built from the source/transform directory,
+which is the filter for the OpenOffice.org XML format.  This legacy format
+is a predecessor of ODF and was the default in OpenOffice.org 1.x versions,
+which did not support ODF.  This filter works as a SAX transformation
+from/to ODF, i.e., when importing a document the transform library reads
+the SAX events from the file and generates SAX events that are then
+consumed by the ODF import filter.
+
+http://www.openoffice.org/xml/general.html
+http://www.openoffice.org/xml/xml_specification.pdf
+
+There is some stuff in the "dtd" directory which is most likely related
+to the OpenOffice.org XML format but is possibly outdated and obsolete.