Update codemirror plugin docs

@pmario and @jbolila - could you kindly check whether the docs cover
everything?

editions/codemirrordemo/tiddlers/HelloThere.tid

@@ -7,3 +7,5 @@ Try editing the tiddler LoremIpsum to try out the editor.
 To add the plugin to your own TiddlyWiki5, just drag this link to the browser window:
 
 [[$:/plugins/tiddlywiki/codemirror]]
+
+{{$:/plugins/tiddlywiki/codemirror/instructions}}

plugins/tiddlywiki/codemirror/instructions.tid

@@ -0,0 +1,20 @@
+title: $:/plugins/tiddlywiki/codemirror/instructions
+
+! Configuration
+
+You can configure the CodeMirror plugin by creating a tiddler called [[$:/config/CodeMirror]] containing a JSON configuration object. For example:
+
+```
+{
+  "require": [
+      "$:/plugins/tiddlywiki/codemirror/addon/dialog.js",
+      "$:/plugins/tiddlywiki/codemirror/addon/searchcursor.js",
+      "$:/plugins/tiddlywiki/codemirror/keymap/vim.js",
+      "$:/plugins/tiddlywiki/codemirror/keymap/emacs.js"
+  ],
+  "configuration": {
+      "keyMap": "vim",
+      "showCursorWhenSelecting": true
+  }
+}
+```