Improve Heroku deployment instructions
Improve Heroku deployment instructions

Updates `README.md`:

* Use abbreviated buildpack names
* Instruct users to add the generated files to version control

seanpdoyle/ember-cli-rails,koryteg/glimmer-rails,rwz/ember-cli-rails,weggesser/ember-cli-rails,seanpdoyle/ember-cli-rails,koryteg/glimmer-rails,seanpdoyle/ember-cli-rails,rwz/ember-cli-rails,thoughtbot/ember-cli-rails,rwz/ember-cli-rails,weggesser/ember-cli-rails,thoughtbot/ember-cli-rails,thoughtbot/ember-cli-rails,weggesser/ember-cli-rails,koryteg/glimmer-rails