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"
 }

plugins/tiddlywiki/highlight/readme.tid

@@ -0,0 +1,5 @@
+title: $:/plugins/tiddlywiki/highlight/readme
+
+This plugin provides syntax highlighting of code blocks using [[highlight.js|https://github.com/isagalaev/highlight.js]] from Ivan Sagalaev.
+
+[[Source code|https://github.com/Jermolene/TiddlyWiki5/blob/master/plugins/tiddlywiki/highlight]]

plugins/tiddlywiki/highlight/usage.tid

@@ -0,0 +1,16 @@
+title: $:/plugins/tiddlywiki/highlight/usage
+
+! Usage
+
+Fenced code blocks can have a language specifier added to trigger highlighting in a specific language. Otherwise heuristics are used to detect the language.
+
+```
+ ```js
+ var a = b + c; // Highlighted as JavaScript
+ ```
+```
+! Adding Themes
+
+You can add themes from highlight.js by copying the CSS to a new tiddler and tagging it with [[$:/tags/Stylesheet]]. The available themes can be found on GitHub:
+
+https://github.com/isagalaev/highlight.js/tree/master/src/styles