add note about pandoc version requirements

Author: gdevanla

README.md

@@ -3,7 +3,9 @@ pandoc-markdown-ghci-filter
 
 A `Pandoc` filter that identifies code blocks(`Haskell`) in Pandoc
 supported formats, executes the code in GHCI and embeds the results in
-the returned output by updating the AST provided by `Pandoc`.
+the returned output by updating the AST provided by `Pandoc`. Note, this
+library is tested with pandoc 2.7.3 and may not be compatible with older
+versions.
 
 Quick Overview
 ==============