Add explanation for Time Sliced Output
Improve outdated task to detect missing documents.
Update the buffer plugin overview

Include a detailed explanation for Time Sliced Plugin
Refactor message when detect not to exist documents.
Update "Detect outdated documents" section of README.
Merge pull request #46 from fluent/feature/improve_outdated_task_for_i18n

Improve "rake outdated" task to detect not to exist articles
Merge pull request #47 from fluent/improve-output-plugin-overview

Improve output plugin overview