README.md: cosmetics
README.md: improve installation instructions

Point users more forcefully into the direction of Elpa and Melpa in
particular.  I am happy to provide other installation methods, but
beginners should really just setup the use of Melpa and be done with
it.
CONTRIBUTING.md: accept reality

Instead of describing Utopia, describe the here and now.  Some of the
removed information might come back later, when it actually matches
reality.
Merge branch 'readme'