Add usage section and a link to the demo page

Author: kazk

README.md

@@ -4,6 +4,20 @@ Lambda Calculus mode for CodeMirror.
 
 - `defineMode(CodeMirror)`: Define `lambdacalc` mode and MIME `text/x-lambdacalc`
 
+## Usage
+
+```javascript
+import CodeMirror from "codemirror";
+
+import { defineMode } from "@codewars/codemirror-lambda-calculus";
+
+defineMode(CodeMirror);
+```
+
+## Demo
+
+See https://codewars.github.io/codemirror-lambda-calculus/
+
 ## Development
 
 Run `npm run dev` to start a local dev server.