Add a preact example. (#543)

* Add preact example.

* Update .babelrc

examples/using-preact/.babelrc

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+{ 
+    "presets": [
+        "es2015",
+        "react"
+    ]
+}

examples/using-preact/README.md

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+
+# Hello World example
+
+## How to use
+
+Download the example (or clone the repo)[https://github.com/zeit/next.js.git]:
+
+```bash
+curl https://codeload.github.com/zeit/next.js/tar.gz/master | tar -xz --strip=2 next.js-master/examples/using-preact
+cd using-preact
+```
+
+Install it and run:
+
+```bash
+npm install
+npm run dev
+```
+
+Deploy it to the cloud with [now](https://zeit.co/now) ([download](https://zeit.co/download))
+
+```bash
+now
+```
+
+## The idea behind the example
+
+This example uses [Preact](https://github.com/developit/preact) instead of React. It's a React like UI framework which fast and small. Here we've customized Next.js to use Preact instead of React.
+
+Here's how we did it:
+
+* Create `.babelrc` file with es2015 and react presets. This allow us to get rid of the hard coded React dependency for core Next.js modules.
+* Use `next.config.js` to customize our webpack config to support [preact-compat](https://github.com/developit/preact-compat)

examples/using-preact/next.config.js

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+module.exports = {
+  webpack: function (config) {
+    config.resolve.alias = {
+      'react': 'preact-compat',
+      'react-dom': 'preact-compat'
+    }
+    return config
+  }
+}

examples/using-preact/package.json

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+{
+  "name": "hello-world",
+  "version": "1.0.0",
+  "scripts": {
+    "dev": "next",
+    "build": "next build",
+    "start": "next start"
+  },
+  "dependencies": {
+    "next": "^2.0.0-beta",
+    "preact": "^7.1.0",
+    "preact-compat": "^3.9.4"
+  },
+  "author": "",
+  "license": "ISC",
+  "devDependencies": {
+    "babel-preset-es2015": "^6.18.0",
+    "babel-preset-react": "^6.16.0"
+  }
+}

examples/using-preact/pages/about.js

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+import React from 'react'
+
+export default () => (
+  <div>About us</div>
+)

examples/using-preact/pages/index.js

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+import React from 'react'
+import Link from 'next/link'
+
+export default () => (
+  <div>Hello World. <Link href='/about'>About</Link></div>
+)