README.rdoc -> README.md for newly generated applications

README.rdoc was generated to support the doc:app task. Now that
this task is gone we can switch to Markdown, which is nowadays
a better default.

railties/CHANGELOG.md

+*   Newly generated applications get a `README.md` in Markdown.
+
+    *Xavier Noria*
+
 *   Remove the documentation tasks `doc:app`, `doc:rails`, and `doc:guides`.
 
     *Xavier Noria*

railties/lib/rails/generators/rails/app/app_generator.rb

@@ -38,7 +38,7 @@ def rakefile
     end
 
     def readme
-      copy_file "README.rdoc", "README.rdoc"
+      copy_file "README.md", "README.md"
     end
 
     def gemfile

railties/lib/rails/generators/rails/app/templates/README.rdoc → railties/lib/rails/generators/rails/app/templates/README.md

-== README
+## README
 
 This README would normally document whatever steps are necessary to get the
 application up and running.

railties/test/generators/actions_test.rb

@@ -222,14 +222,14 @@ def test_route_should_add_data_to_the_routes_block_in_config_routes
   def test_readme
     run_generator
     Rails::Generators::AppGenerator.expects(:source_root).times(2).returns(destination_root)
-    assert_match "application up and running", action(:readme, "README.rdoc")
+    assert_match "application up and running", action(:readme, "README.md")
   end
 
   def test_readme_with_quiet
     generator(default_arguments, quiet: true)
     run_generator
     Rails::Generators::AppGenerator.expects(:source_root).times(2).returns(destination_root)
-    assert_no_match "application up and running", action(:readme, "README.rdoc")
+    assert_no_match "application up and running", action(:readme, "README.md")
   end
 
   def test_log

railties/test/generators/app_generator_test.rb

@@ -5,7 +5,7 @@
 
 DEFAULT_APP_FILES = %w(
   .gitignore
-  README.rdoc
+  README.md
   Gemfile
   Rakefile
   config.ru