Add info about env vars in README

Without HUBOT_ADAPTER, Hubot crashes silently on Heroku:

    2013-12-29T13:47:38.585886+00:00 heroku[web.1]: State changed from
    crashed to starting
    2013-12-29T13:47:46.265427+00:00 heroku[web.1]: State changed from
    starting to crashed