Docs: add "Hello World" tutorial.

docs/guide/hello-world.md

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+First [get setup with Calypso locally][1] if you haven't already...
+
+Load [http://calypso.localhost:3000][2] in your browser. All good? Cool, now let's build our _Hello World_.
+
+# Adding a new section
+
+Creating a new section is composed of five steps:
+
+1. Add your feature `config/development.json`.
+2. Setup your section folder.
+3. Create a controller `client/my-sites/my-section/controller.js`.
+4. Setup the entry routes in `client/my-sites/my-section/index.js`.
+5. Register section in `client/sections.js.`
+
+### 1. Add feature
+
+First thing is to enable your new feature in Calypso. We'll do that by opening `config/development.json` and adding a new feature entry in `features {}`:
+
+```
+"hello-world": true
+```
+
+### 2. Set up folder structure
+
+For the sake of this tutorial, we are creating a _Hello World_ page within the my-sites section. Create a directory called `hello-world` in `client/my-sites`.
+
+You can run this command on your terminal within your local Calypso project:
+
+```
+mkdir client/my-sites/hello-world
+```
+
+### 3. Create a controller file
+
+The `controller.js` file is where you can find the various functions that get called at each specific route. Let's add our own!
+
+```
+touch client/my-sites/hello-world/controller.js
+```
+
+There you'll write your controller with a function called `helloWorld`:
+
+```javascript
+const Controller = {
+	helloWorld() {
+		console.log( 'Hello, world?' );
+	}
+};
+
+export default Controller;
+```
+
+### 4. Setup the route
+
+Next step is to create the main file for your section, called `index.js` within `hello-world`. Run the following command:
+
+```
+touch client/my-sites/hello-world/index.js
+```
+
+Here we'll require the `page` module, the My Sites controller, our own controller, and write our main route handler:
+
+```javascript
+/**
+ * External dependencies
+ */
+import page from 'page';
+
+/**
+ * Internal dependencies
+ */
+import controller from 'my-sites/controller';
+import helloWorldController from './controller';
+
+export default () => {
+	page( '/hello-world/:domain?', controller.siteSelection, controller.navigation, helloWorldController.helloWorld );
+};
+```
+
+`page()` will set up the route `/hello-world` and run some functions when it's matched. The `:domain?` is because we want to support site specific pages for our hello-world route. We are passing the `siteSelection` function from the main "My Sites" controller, which handles the site selection process. The last function is our newly created controller handler.
+
+### 4. Register section
+
+Now it's time to configure our section. Open `client/my-sites/sections.js` and add the following code:
+
+```javascript
+if ( config.isEnabled( 'hello-world' ) ) {
+	sections.push( {
+		name: 'hello-world',
+		paths: [ '/hello-world' ],
+		module: 'my-sites/hello-world'
+	} );
+}
+```
+
+This checks for our feature in the current environment to figure out whether it needs to register a new section. The section is defined by a name, an array with the relevant paths, and the main module.
+
+### Run the server!
+
+Restart the server doing:
+
+* `make clean`
+* `make run`
+
+We are ready to load `calypso.localhost:3000/hello-world`! Your console should respond with `Hello, world?` if everything is working and you should see Calypso's sidebar for "My Sites".
+
+----
+
+# The View
+
+Now let's build our main view using a React component. For this task we have two steps:
+
+1. Create JSX file called `main.jsx` in `client/my-sites/hello-world`.
+2. Hook up controller.
+
+
+### 1. Create main view
+
+Create an empty file `main.jsx` by running the following command:
+
+```
+touch client/my-sites/hello-world/main.jsx
+```
+
+Start by importing React as an external dependency at the top, then import the internal "Main" UI component from `components/main`. We'll use it to set up our view tree. Finally, create a new React class and set it up with the feature name as its "displayName":
+
+```javascript
+/**
+ * External dependencies
+ */
+import React from 'react';
+
+/**
+ * Internal dependencies
+ */
+import Main from 'components/main';
+
+export default React.createClass( {
+	displayName: 'HelloWorld',
+} );
+```
+
+Cool. Let's make the React component render something for us. We'll do that by adding a `render()` method that uses the "Main" component and outputs some markup. Inside the `React.createClass` object, after "displayName", write:
+
+```javascript
+render() {
+	return (
+		<Main>
+			<h1>Hello, World!</h1>
+		</Main>
+	);
+}
+```
+
+### 2. Hook up controller
+
+Time to hook this up with our controller function. Open `/hello-world/controller.js`. Import React again at the top of the file, then remove the `console.log` call and enter the following instead:
+
+```javascript
+helloWorld() {
+	const Main = require( 'my-sites/hello-world/main' );
+
+	// Render hello world...
+	React.render(
+		React.createElement( Main ),
+		document.getElementById( 'primary' )
+	);
+}
+```
+
+In the `Main` constant we are getting our main jsx file for our section. We then instruct React to render `Main` in `#primary`, an element that is already on the DOM.
+
+(If you want to see where `#primary` is created open `client/layout/index.jsx`.)
+
+### Ok, ready?
+
+Run `make run` if it wasn't already running, and load [http://calypso.localhost:3000/hello-world][3] in your browser. You should see "Hello, World!" on the page next to the sidebar. And since we added `controller.siteSelection` in our initial route setup, changing a site in the sidebar should also work for your hello-world section. Happy _calypsoing_!
+
+[1]: https://github.com/Automattic/wp-calypso#installing-and-running
+[2]: http://calypso.localhost:3000
+[3]: http://calypso.localhost:3000/hello-world