Add link to playground in README

pkgs/markdown/README.md

diff --git a/pkgs/markdown/README.md b/pkgs/markdown/README.md
index d661973..75f62e0 100644
--- a/pkgs/markdown/README.md
+++ b/pkgs/markdown/README.md
@@ -3,6 +3,9 @@
 A portable Markdown library written in Dart. It can parse Markdown into
 HTML on both the client and server.
 
+Play with it at
+[dart-lang.github.io/markdown](https://dart-lang.github.io/markdown).
+
 ### Usage
 
 ```dart