docs: Declare planned spec

README.rst

@@ -2,7 +2,17 @@
 atsphinx-reveal-og-image
 ========================
 
-Generate og:image content using Reveal.js
+Generate ``og:image`` content using Reveal.js
+
+Overview
+========
+
+This is Sphinx-extension to generate image contents for ``og:image``.
+
+Images are generated from title of documents and project name
+using Reveal.js presentation with Playwright.
+
+.. note:: Installation size is large although only purpose of generating images.
 
 Getting started
 ===============
@@ -17,3 +27,7 @@ Getting started
        ...,  # Your extensions
        "atsphinx.reveal_og_image",
    ]
+
+.. This is plan.
+
+   When you build document, this generates images into ``BUILD_DIR/_images/og-image/{document-name}.png``