Improve plugin readmes

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

plugins/tiddlywiki/highlight/plugin.info

@@ -2,5 +2,6 @@
 	"title": "$:/plugins/tiddlywiki/highlight",
 	"description": "Highlight.js: syntax highlighting",
 	"author": "JoaoBolila",
-	"list": "license"
+	"core-version": ">=5.0.0",
+	"list": "readme usage license"
 }