Adds the Rails maintenance policy to the Guides

I propose that we add the Rails maintenance policy to the Ruby on Rails Guides.

At the moment if you want to check the maintenance policy you have to find [the blog post about it way from back in February](http://weblog.rubyonrails.org/2013/2/24/maintenance-policy-for-ruby-on-rails/) and it doesn't seem to have been updated after Rails 4 was released.

Ideally Rails Guides should be the canonical source for the maintenance policy (and I think there would be a better chance of it staying up-to-date from release to release)…

I've done my best to update the maintenance policy to the current state from the blog post from February and I'm fairly certain that it reflects reality, but someone more knowledgeable should check it to be sure.