🤖 Add rtd-bot (#243)

_apps/rtd-helper.md

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+---
+# A human-friendly name of your listing
+title: Read The Docs Helper
+# A short description of what your app does
+description: Assists reviewing using Read The Docs (RTD)
+# The slug of your hosted app on GitHub (https://github.com/apps/YOUR-SLUG)
+slug: rtd-helper
+# Include a few screenshots that show your app in action
+screenshots:
+- https://raw.githubusercontent.com/KengoTODA/rtd-bot/master/screenshot.png
+# The GitHub usernames of anyone who authored the app
+authors:
+- KengoTODA
+# The repository where the code is located
+repository: KengoTODA/rtd-bot
+# The address where this app is deployed
+host: https://rtd-bot.herokuapp.com
+---
+
+Get URL of documents when review changes in docs directory.
+
+Work with Read The Docs, then you'll find that PR for documentation needs additional steps like:
+
+* running RTD build for your branch manually, to use its result as staging site [like this](https://github.com/spotbugs/spotbugs/pull/697#issue-201455071), or
+* sharing screenshot to share the updated document [like this](https://github.com/spotbugs/spotbugs/pull/718#issue-205904835).
+
+This bot automates the first approach; activate RTD build automatically when you made PR that updates `docs/` directory.