5.3 Creating Posts

To make the "Unknown action" go away, you can define a create action
within the PostsController class in app/controllers/posts_controller.rb,
underneath the new action:
class PostsController < ApplicationController
  def new
  end

  def create
  end
end

If you re-submit the form now, you'll see another familiar error: a
template is missing. That's ok, we can ignore that for now.