Initial compilation of release notes for 2.5

This is a first real attempt at compiled release notes for 2.5.

A few things to note:
1) While content was moved around a little (New Features vs Upgrade
Notes), nothing was outright removed.
2) Some of the sections are indented using ':leveloffset:'.  As a
result, some section headers end up being more than 5 levels deep, and
therefore do not display when generating (for example) HTML.  I think
(in most cases) we want to trim these parts out of this 'release notes'
file and put the offending sections in the actual documentation for
the feature, so I am leaving it as-is for now.

To easily see these too-deep sections, you can run:

asciidoc -a toc -a numbered RELEASE_NOTES_2_5.txt

and look at the warnings.

I have also not yet cleared out the RELEASE_NOTES_NEXT folder, as I
want to ask some of the docs folks first before I do so.

Signed-off-by: Dan Wells <dbw2@calvin.edu>