Update README.md

Suggest not requiring the gem by default, as we have omitted the `lib/capistrano-slackify.rb` file that can avoid auto-require issues.  Internally we haven't run into this because we require this gem via our shared deploy gem.
Add empty file to resolve auto-require issue

Resolves #15
Prepare for v2.0.1 release
Update README.md
Update README.md
Add `slack:notify_failed` task

Create slack notifications for deployment failures using the
`deploy:failed` hook (`v3.1.0`) and a `:slack_deploy_failed_text` variable.