Recod version of markdown2 that works.

Versions > 2.3.3 of of markdown2 has a bug that causes
[issue1](issue1) to render as <a href="#"> rather than <a
href="issue1">. Up till 2.4.9, Roundup wasn't afected by the bug.

As of 2.4.9 the bug is causing issues.

Document that this version should not be used.

See: trentm/python-markdown2#517

Author: rouilj

doc/installation.txt

@@ -256,7 +256,8 @@ docutils
   package installed.
 
 markdown, markdown2 or mistune
-  To use markdown rendering you need to have the markdown_, markdown2_
+  To use markdown rendering you need to have the markdown_,
+  markdown2_ (2.4.9 known to be broken, 2.3.3 known to work),
   or mistune_ (v0.8.4 tested) package installed.
 
 zstd, brotli