Add first pass of help panel

This is just an idea really.

The help plugin is only included in the tw5.com wiki; the idea is to
also include it in the empty.html wiki, either as a plugin or as part
of the core.

If it stays as a plugin, the idea would be to have different help
plugins for different languages.

plugins/tiddlywiki/help/plugin.info

@@ -0,0 +1,7 @@
+{
+	"title": "$:/plugins/tiddlywiki/help",
+	"description": "Floating help window for TiddlyWiki",
+	"author": "JeremyRuston",
+	"core-version": ">=5.0.0",
+	"list": "readme"
+}