Release notes: links in intro

Bruce, Robert,

This patch adds a bunch of feature links to other parts of the docs to
the "big features" portion of the release notes.  I was unable to find
an appropriate link for ORDER BY in aggregates; please fill in if you
know of one.

If there's something wrong with the way I pulled the patch, just let me
know and I'll fix it.

--
                                   -- Josh Berkus
                                      PostgreSQL Experts Inc.
                                      http://www.pgexperts.com