Add `RAILS_ENV` subsection to README.md
Add `RAILS_ENV` subsection to README.md

The `RAILS_ENV` documentation stands on its own and thus deserves its
own section.

koryteg/glimmer-rails,koryteg/glimmer-rails,thefrontside/ember-cli-rails,johan--/ember-cli-rails,thoughtbot/ember-cli-rails,thoughtbot/ember-cli-rails,harisadam/ember-cli-rails,weggesser/ember-cli-rails,seanpdoyle/ember-cli-rails,rwz/ember-cli-rails,rwz/ember-cli-rails,seanpdoyle/ember-cli-rails,rwz/ember-cli-rails,macroplant/ember-cli-rails,thoughtbot/ember-cli-rails,seanpdoyle/ember-cli-rails,jgadbois/ember-cli-rails,koryteg/glimmer-rails,weggesser/ember-cli-rails,KeenRegard/ember-cli-rails,Szeliga/ember-cli-rails,jesenko/ember-cli-rails,weggesser/ember-cli-rails