Add readme for strikethrough extension

markdown/extension/gfm-strikethrough/README.md

@@ -0,0 +1,41 @@
+# GitHub Flavored Markdown — strikethrough extension
+
+This extension adds support for strikethrough,
+a [GFM extension](https://github.github.com/gfm/#strikethrough-extension-) over
+CommonMark. Strikethrough nodes are parsed by the `commonmark-ext-gfm-strikethrough` library, and rendered simply by
+wrapping their content in a `SpanStyle` that applies the `LineThrough` decoration.
+
+![Screenshot of a strikethrough](../../../art/docs/gfm-strikethrough.png)
+
+## Usage
+
+To use the strikethrough extension, you need to add the `GitHubStrikethroughProcessorExtension` to your
+`MarkdownProcessor`, and the
+`GitHubStrikethroughRendererExtension` to the `MarkdownBlockRenderer`. For example, in standalone mode:
+
+```kotlin
+val isDark = JewelTheme.isDark
+
+val markdownStyling = remember(isDark) { if (isDark) MarkdownStyling.dark() else MarkdownStyling.light() }
+
+val processor = remember { MarkdownProcessor(listOf(GitHubStrikethroughProcessorExtension)) }
+
+val blockRenderer =
+    remember(markdownStyling) {
+        if (isDark) {
+            MarkdownBlockRenderer.dark(
+                styling = markdownStyling,
+                rendererExtensions = listOf(GitHubStrikethroughRendererExtension),
+            )
+        } else {
+            MarkdownBlockRenderer.light(
+                styling = markdownStyling,
+                rendererExtensions = listOf(GitHubStrikethroughRendererExtension),
+            )
+        }
+    }
+
+ProvideMarkdownStyling(markdownStyling, blockRenderer, NoOpCodeHighlighter) {
+    // Your UI that renders Markdown goes here
+}
+```

markdown/extension/gfm-strikethrough/build.gradle.kts

@@ -16,3 +16,7 @@ publishing.publications.named<MavenPublication>("main") {
     val ijpTarget = project.property("ijp.target") as String
     artifactId = "jewel-markdown-extension-${project.name}-$ijpTarget"
 }
+
+publicApiValidation {
+    excludedClassRegexes = setOf("org.jetbrains.jewel.markdown.extensions.github.strikethrough.*")
+}