diff --git a/README b/README
index 4b27f3dde..094ced539 100644
--- a/README
+++ b/README
@@ -2207,6 +2207,13 @@ Causes anything between `\\(` and `\\)` to be interpreted as inline
 TeX math, and anything between `\\[` and `\\]` to be interpreted
 as display TeX math.
 
++**Extension: `markdown_attribute`**\
+Causes the attribute `markdown=1` to be added to all block-level
+HTML tags that might contain markdown.  In pandoc, material inside
+block-level tags is interpreted a markdown by default, but in some
+other implementations, the `markdown=1` tag is needed.
+
+
 Producing slide shows with Pandoc
 =================================