Add the beginnings of a Sky Widgets README.md

There's lots more ground to cover, but this CL is a start.

TBR=ianh@google.com

Review URL: https://codereview.chromium.org/1187173003.

sdk/lib/widgets/README.md

+Sky Widgets
+===========
+
+Sky widgets are built using a functional-reactive framework, which takes
+inspiration from [React](http://facebook.github.io/react/). The central idea is
+that you build your UI out of components. Components describe what their view
+should look like given their current configuration and state. When a component's
+state changes, the component rebuilds its description, which the framework diffs
+against the previous description in order to determine the minial changes needed
+in the underlying render tree to transition from one state to the next.
+
+Hello World
+-----------
+
+To build an application, create a subclass of App and instantiate it:
+
+```dart
+import 'package:sky/widgets/basic.dart';
+
+class HelloWorldApp extends App {
+  Widget build() {
+    return new Text('Hello, world!');
+  }
+}
+
+void main() {
+  new HelloWorldApp();
+}
+```
+
+An app is comprised of (and is, itself, a) widgets. The most commonly authored
+widgets are, like `App`, subclasses of `Component`.  A component's main job is
+to implement `Widget build()` by returning newly-created instances of other
+widgets. If a component builds other components, the framework will build those
+components in turn until the process bottoms out in a collection of basic
+widgets, such as those in `sky/widgets/basic.dart`. In the case of
+`HelloWorldApp`, the `build` function simply returns a new `Text` node, which is
+a basic widget representing a string of text.
+
+Basic Widgets
+-------------
+
+Sky comes with a suite of powerful basic widgets, of which the following are
+very commonly used:
+
+ * `Text`. The `Text` widget lets you create a run of styled text within your
+   application.
+
+ * `Flex`. The `Flex` widget lets you create flexible layouts in both the
+   horizontal and vertical direction. Its design is based on the web's flexbox
+   layout model. You can also use the simpler `Block` widget to create vertical
+   layouts of inflexible items.
+
+ * `Container`. The `Container` widget lets you create rectangular visual
+   element. A container can be decorated with a `BoxDecoration`, such as a
+   background, a border, or a shadow. A `Container` can also have margins,
+   padding, and constraints applied to its size. In addition, a `Container` can
+   be transformed in three dimensional space using a matrix.
+
+ * `Image`. The `Image` widget lets you display an image, referenced using a
+   URL. The underlying image is cached, which means if several `Image` widgets
+   refer to the same URL, they'll share the underlying image resource.
+
+Below is a simple toolbar example that shows how to combine these widgets:
+
+```dart
+import 'package:sky/widgets/basic.dart';
+
+class MyToolBar extends Component {
+  Widget build() {
+    return new Container(
+      child: new Flex([
+        new Image(src: 'menu.png', size: const Size(25.0, 25.0)),
+        new Flexible(child: new Text('My awesome toolbar')),
+        new Image(src: 'search.png', size: const Size(25.0, 25.0)),
+      ]),
+      height: 56.0,
+      padding: new EdgeDims.symmetric(horizontal: 8.0),
+      decoration: const BoxDecoration(
+        backgroundColor: const Color(0xFF00FFFF),
+      )
+    );
+  }
+}
+```
+
+The `MyToolBar` component creates a cyan `Container` with a height of 56
+device-independent pixels with an internal padding of 8 pixels, both on the
+left and the right. Inside the container, `MyToolBar` uses a `Flex` layout
+in the (default) horizontal direction. The middle child, the `Text` widget, is
+marked as `Flexible`, which means it expands to fill any remaining available
+space that hasn't been consumed by the inflexible children. You can have
+multiple `Flexible` children and determine the ratio in which they consume the
+available space using the `flex` argument to `Flexible`.
+
+To use this component, we simply create an instance of `MyToolBar` in a `build`
+function:
+
+```dart
+import 'package:sky/widgets/basic.dart';
+
+import `my_tool_bar.dart';
+
+class DemoApp extends App {
+  Widget build() {
+    return new Center(child: new MyToolBar());
+  }
+}
+
+void main() {
+  new DemoApp();
+}
+```
+
+Here, we've used the `Center` widget to center the toolbar within the view, both
+vertically and horizontally. Because the toolbar uses a flexible layout
+internally, it expands to fill all the available horizontal space.
+
+Listening to Events
+-------------------
+
+In addition to being stunningly beautiful, most applications react to user
+input. The first step in building an interactive application is to listen for
+input events. Let's see how that works by creating a simple button:
+
+```dart
+import 'package:sky/widgets/basic.dart';
+
+final BoxDecoration _decoration = new BoxDecoration(
+  borderRadius: 5.0,
+  gradient: new LinearGradient(
+    endPoints: [ Point.origin, new Point(0.0, 36.0) ],
+    colors: [ const Color(0xFFEEEEEE), const Color(0xFFCCCCCC) ]
+  )
+);
+
+class MyButton extends Component {
+  Widget build() {
+    return new Listener(
+      child: new Container(
+        child: new Center(
+          child: new Text('Engage')
+        ),
+        height: 36.0,
+        padding: new EdgeDims.all(8.0),
+        margin: new EdgeDims.symmetric(horizontal: 8.0),
+        decoration: _decoration
+      ),
+      onGestureTap: (event) {
+        print('MyButton was tapped!');
+      }
+    );
+  }
+}
+```
+
+The `Listener` widget doesn't have an visual representation but instead listens
+for events bubbling through the application. When a tap gesture bubbles out from
+the `Container`, the `Listener` will call its `onGestureTap` callback, in this
+case printing a message to the console.
+
+You can use `Listener` to listen for a variety of input events, including
+low-level pointer events and higher-level gesture events, such as taps, scrolls,
+and flings.
+
+Generic Components
+------------------
+
+One of the most powerful features of components is the ability to pass around
+references to already-built widgets and reuse them in your `build` function. For
+example, we wouldn't want to define a new button component every time we wanted
+a button with a novel label:
+
+```dart
+class MyButton extends Component {
+  MyButton({ this.child, this.onPressed });
+
+  final Widget child;
+  final Function onPressed;
+
+  Widget build() {
+    return new Listener(
+      child: new Container(
+        child: new Center(child: child),
+        height: 36.0,
+        padding: new EdgeDims.all(8.0),
+        margin: new EdgeDims.symmetric(horizontal: 8.0),
+        decoration: _decoration
+      ),
+      onGestureTap: (_) {
+        if (onPressed != null)
+          onPressed();
+      }
+    );
+  }
+}
+```
+
+Rather than providing the button's label as a `String`, we've let the code that
+uses `MyButton` provide an arbitrary `Widget` to put inside the button. For
+example, we can put an elaborate layout involving text and an image inside the
+button:
+
+```dart
+  Widget build() {
+    return new MyButton(
+      child: new ShrinkWrapWidth(
+        child: new Flex([
+          new Image(src: 'thumbs-up.png', size: const Size(25.0, 25.0)),
+          new Container(
+            child: new Text('Thumbs up'),
+            padding: new EdgeDims.only(left: 10.0)
+          )
+        ])
+      )
+    );
+  }
+```
+
+State
+-----
+
+TODO(abarth)