adding notification for rdoc

Author: tenderlove

railties/lib/tasks/documentation.rake

@@ -1,3 +1,5 @@
+begin
+  require 'rdoc/task'
 namespace :doc do
   desc "Generate documentation for the application. Set custom template with TEMPLATE=/path/to/rdoc/template.rb or title with TITLE=\"Custom Title\""
   RDoc::Task.new("app") { |rdoc|
@@ -86,3 +88,6 @@ namespace :doc do
     end
   end
 end
+rescue LoadError
+  $stderr.puts 'Please install RDoc 2.4.2+ to generate documentation.'
+end