Encrypt credentials (Rails 5.2) / secrets (Rails 5.1) in guidelines

[eliotsykes]

Rails 5.2 is looking like it will replace encrypted secrets with encrypted credentials - see PR here: rails/rails#30067

Rails 5.1 was released with an encrypted secrets feature: rails/rails#25095 + PR: rails/rails#28038

Recommend this in the checklist and possibly its alternatives for earlier Rails versions (e.g. sekrets gems mentioned in rails/rails#25095)

[eliotsykes]

Overview of encrypted secrets, include link? https://www.engineyard.com/blog/encrypted-rails-secrets-on-rails-5.1