Add task for building rails edge docs

Closes rails#81

Rakefile

@@ -11,3 +11,24 @@ end
 
 task :default => :test
 task :spec => :test
+
+require 'sdoc'
+require 'rdoc/task'
+
+rails = File.expand_path "rails"
+
+directory rails do
+  sh "git clone --depth=1 https://github.com/rails/rails"
+end
+
+namespace :test do
+  task :rails => rails
+
+  RDoc::Task.new(:rails) do |rdoc|
+    rdoc.rdoc_dir = 'doc/rails'
+    rdoc.generator = 'sdoc'
+    rdoc.template = 'rails'
+
+    rdoc.rdoc_files.include("rails/")
+  end
+end