Add task with docs for builtin generation

tasks/generate/builtins.rake

@@ -0,0 +1,13 @@
+namespace :generate do
+  desc "Generate the builtins for the given language"
+  task :builtins, [:lang] do |t, args|
+    exit unless args.lang
+
+    builtin_task = "builtins:#{args.lang.downcase}"
+    if Rake::Task.task_defined? builtin_task
+      Rake::Task[builtin_task].invoke
+    else
+      puts "There is no builtin generator for #{args.lang}"
+    end
+  end
+end