Update README.md

Specify `bundle exec` with testing example. Sometimes running without bundle exec works, but not always, and might be confusing to non-rubyists.
Merge pull request #4 from amiel/patch-1

Update README.md
test that message can be set from another controller
Docs for flash message from controller
rebuild
clean up readme
Allow this.flashMessage from controllers

Closes #2, even though it is already closed.