Improve plugin readmes

Now every plugin has a short, introductory readme tiddler that is shown
in the online plugin library.

plugins/tiddlywiki/markdown/plugin.info

@@ -1,5 +1,7 @@
 {
 	"title": "$:/plugins/tiddlywiki/markdown",
 	"description": "Markdown parser",
-	"author": "JeremyRuston"
+	"author": "JeremyRuston",
+	"core-version": ">=5.0.0",
+	"list": "readme usage"
 }