some comments about the usefulnes of the OOXML SDK for development

Hey,

so after I noticed at the Hamburg hackfest that apparently not
everybody knows about the benefits of the OOXML SDK I'll write here a
quick summary of where it helps during development.

The OOXML SDK has the following nice features that help during work on
the filters:

* Validation of files
* MSO implementers notes for OOXML
* Validation against the MSO schemas and not the OOXML schemas ( most
of the time this is a feature as it helps to find out why MSO rejects
your valid files )
* Navigation through the xml elements in a window with automatic link
to the file format spec
* Documentation of at least some extLst entries and maybe other MSO
extensions to the file format (extremely helpful as I found no other
documentation of them)

I hope that these information might help some people as much as they
helped me already when I had to understand why my files are not opened
correctly in Excel.

Regards,
Markus