Update README
Update README

 * Tighten intro sentence
 * Explicitly mention Dokku 0.5
 * Reference new docker image for letsencrypt-simp_le
 * Remove warnings about update problems (should be fixed by now)
 * Remove warnings about configuration changes (most people have updated by now)

dokku/dokku-letsencrypt,sseemayer/dokku-letsencrypt