These versions provide greatly desired performance advantages for
notmuch.
Previously, theses details existed in an old NEWS entry, but most
users are unlikely to find those details there. Put them here where we
mention the Xapian dependency.
Write down the steps needed to install and actuall use notmuch in emacs. Should
help emacs newbies.
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>