Landing page for SDW documents on w3c.github.io

Hi- to make it easy for folks to find the documents we're working on, I
have added a landing page [1] to our publication location on GitHub Pages.

This is modelled on what we did for the CSV on the Web WG.

As we add more working documents (e.g. BP doc) we can add links. We can
also list the list of (FP)WDs as they are published. It would seem
appropriate to _also_ list the OGC document number and link their artifact
for each (FP)WD too.

@kerry: you may want to update with a link that points to you (see where
the WG chairs are listed).

Hope this meets approval of the group ... if not we can just remove it :-)

Jeremy

[1]: http://w3c.github.io/sdw/index.html