A link to the README of each module is included in "Welcome to Rails",
only ActiveModel (and ActiveJob, see #17264) are missing.

[ci skip]

[ci skip]

Ruby on Rails Tutorial link updated.

Documented enqueue_at and enqueue_in

We will add it when a stable version is out.

beanstalkd and rabbitmq are MQ servers, they aren't something that would integrate directly with Rails and ActiveJob.  That's like saying we want an adapter for Redis, when really you want adapters for Resque, Sidekiq, etc.

Sneakers is the latest Ruby job system using rabbitmq.  I don't know of a modern beanstalkd system for Ruby, Stalker is pretty dead these days.  I added QueueClassic because it's reasonably popular on Heroku for the "postgresql only" crowd.

We tend to use Markdown for everything but the API, the
README is no longer used as the home page of the API
after 7a5aa720, so we can now migrate it.

* Organized code status badges into a "Code Status" section.
* Moved the OSI MIT License link inline (bullets are for unordered
  lists).
* Removed all EOL whitespace.

[ci skip]