Add description for retab task

lib/tasks/whitespace.rake

@@ -3,6 +3,7 @@ namespace :whitespace do
   task :cleanup do
     sh %{find . -name '*.rb' -exec sed -i '' 's/ *$//g' {} \\;}
   end
+  desc 'Converts hard-tabs into two-space soft-tabs'
   task :retab do
     sh %{find . -name '*.rb' -exec sed -i '' 's/\t/  /g' {} \\;}
   end