Initial release notes created for 9.6

I've pushed a first cut at release notes for 9.6.  There's a good deal
of work to do yet:

* The section about parallel query could probably stand to be fleshed out,
but I'm unsure what to say.  Somebody who's worked on that should provide
some text.

* Bruce usually likes to sprinkle the notes with a whole lot of links
to the main docs.  I've only bothered with links for new GUCs and system
views.

* As is somewhat customary for early drafts of the notes, I've made no
attempt to call out which are the most significant changes.  I've not
tried to isolate the non-backwards-compatible items, either.

Please review and comment before Monday, if you can.
        regards, tom lane