docs: add demo for React hooks and context (#150)

demo/reactjs/src/AppWithHooks.js

@@ -0,0 +1,41 @@
+/*
+ * function Component with Hooks: Modify ./index.js to apply this file
+ */
+import React, { useState, useLayoutEffect } from 'react';
+import './App.css';
+import logo from './logo.svg';
+import tplsrc from './views/showing-click-times.liquid';
+import Parser from 'html-react-parser';
+import { engine } from './engine';
+import { Context } from './Context';
+import { ClickButton } from './ClickButton';
+
+const fetchTpl = engine.getTemplate(tplsrc.toString())
+
+export function App() {
+  const [state, setState] = useState({
+    logo: logo,
+    name: 'alice',
+    clickCount: 0,
+    html: ''
+  });
+
+  useLayoutEffect(() => {
+    fetchTpl
+      .then(tpl => engine.render(tpl, state))
+      .then(html => setState({...state, html}))
+  }, [state.clickCount])
+
+  return (
+    <div className="App">
+      {Parser(`${state.html}`)}
+      <Context.Provider
+        value={{
+          count: () => setState({...state, clickCount: state.clickCount + 1})
+        }}
+      >
+        <ClickButton/>
+      </Context.Provider>
+    </div>
+  );
+}