[NaCl Docs] Add release-notes.rst.

This is direct port of the current devsite content.

I've only ports the notes as far back as pepper_22.
The rest should probably get ported as well but I'm
landing this so we have a useful release notes page
to start with.

R=eliben@chromium.org

Review URL: https://codereview.chromium.org/26624002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228227 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228227