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"
 }

plugins/tiddlywiki/markdown/readme.tid

@@ -0,0 +1,7 @@
+title: $:/plugins/tiddlywiki/markdown/readme
+
+This is a TiddlyWiki plugin for parsing Markdown text, based on the [[markdown-js|https://github.com/evilstreak/markdown-js]] project from Dominic Baggott. 
+
+It is completely self-contained, and doesn't need an Internet connection in order to work. It works both in the browser and under Node.js.
+
+[[Source code|https://github.com/Jermolene/TiddlyWiki5/blob/master/plugins/tiddlywiki/markdown]]

plugins/tiddlywiki/markdown/usage.tid

@@ -0,0 +1,30 @@
+title: $:/plugins/tiddlywiki/markdown/usage
+
+! Markdown Dialects
+
+By default the markdown parser recognises the original dialect of Markdown [[as described by John Gruber|http://daringfireball.net/projects/markdown/]]. An extended dialect called "Maruku" is also included that provides table support and other advanced features. The syntax extensions are modelled on those of [[PHP Markdown Extra|https://michelf.ca/projects/php-markdown/extra/]].
+
+The configuration tiddler [[$:/config/markdown/dialect]] determines which dialect is used:
+
+|!Dialect |!Description |
+|Gruber |Standard Markdown |
+|Maruku |Extended Maruku Markdown |
+
+
+! Creating ~WikiLinks
+
+Create wiki links with the usual Markdown link syntax targeting `#` and the target tiddler title:
+
+```
+[link text](#TiddlerTitle)
+```
+
+! Images
+
+Markdown image syntax can be used to reference images by tiddler title or an external URI. For example:
+
+```
+![alt text](/path/to/img.jpg "Title")
+
+![alt text](Motovun Jack.jpg "Title")
+```