Update documentation in preparation for 0.12

Now that `deisctl` has landed, I've run through another end-to-end test with an eye toward documentation.  The following are included in this PR:

 * Reference all 3 deployment workflow (and provide links to proper docs)
 * Use only the Heroku Buildpack workflow in the README
 * Add documentation for "Using Docker Images" which was missing from `deis pull` PR
 * Minor language updates
I should have changed this when I edited the Makefile. You can just do `make discovery-url` now.