Describe pymarkdown extensions

build.py

@@ -20,7 +20,16 @@ def main():
     src = pathlib.Path("src")
     out = pathlib.Path(args.out)
 
-    md = markdown.Markdown(extensions=["attr_list", "footnotes", "md_in_html", "meta"])
+    md = markdown.Markdown(extensions=[
+        # Set HTML attributes with {#id}
+        "attr_list",
+        # Footnotes [^1]
+        "footnotes",
+        # Parse markdown within HTML[markdown] blocks
+        "md_in_html",
+        # "YAML" frontmatter metadata
+        "meta",
+    ])
     comment_md = markdown.Markdown()
 
     # Map of feed url -> FeedGenerator object