the main page of the API generated by the doc:rails task is README

Author: fxn

railties/lib/rails/tasks/documentation.rake

@@ -55,7 +55,7 @@ namespace :doc do
     rdoc.template = "#{ENV['template']}.rb" if ENV['template']
     rdoc.title    = "Rails Framework Documentation"
     rdoc.options << '--line-numbers' << '--inline-source'
-    rdoc.rdoc_files.include('README.rdoc')
+    rdoc.rdoc_files.include('README')
 
     gem_path('actionmailer') do |actionmailer|
       %w(README.rdoc CHANGELOG MIT-LICENSE lib/action_mailer/base.rb).each do |file|