Add content to the API doc landing page (#24193)

dev/docs/README.md

+Welcome to the Flutter API reference documentation.
+
 Flutter is Google’s mobile UI framework for crafting high-quality native
 interfaces on iOS and Android in record time. Flutter works with existing code,
 is used by developers and organizations around the world, and is free and open
 source.
 
-### Documentation
+The API reference herein covers all libraries that are exported by the Flutter
+SDK.
+
+### More Documentation
 
 This site hosts Flutter's API documentation. Other documentation can be found at
 the following locations:
 
-* **Main site: [flutter.io](https://flutter.io/)**
+* [flutter.io](https://flutter.io/) (main site)
 * [Installation](https://flutter.io/docs/get-started/install)
 * [Codelabs](https://flutter.io/docs/codelabs)
 * [Contributing to Flutter](https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md)
+
+### Importing a Library
+
+#### Framework Libraries
+
+Libraries in the "Libraries" section below (or in the left navigation) are part
+of the core Flutter framework and are imported using
+`'package:flutter/<library>.dart'`, like so:
+
+```dart
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+```
+
+#### Dart Libraries
+
+Libraries in the "Dart" section exist in the `'dart:'` namespace and are imported
+using `'dart:<library>'`, like so:
+
+```dart
+import 'dart:async';
+import 'dart:ui';
+```
+
+Except for `'dart:core'`, you must import a Dart library before you can use it.
+
+#### Other Libraries
+
+Libraries in other sections are supporting libraries that ship with Flutter.
+They are organized by package and are imported using
+`'package:<package>/<library>.dart'`, like so:
+
+```dart
+import 'package:flutter_test/flutter_test.dart';
+import 'package:file/local.dart';
+```
+
+### Finding Other Libraries
+
+Flutter has a rich community of packages that have been contributed by the
+open-source community. You can browse those packages at
+[pub.dartlang.org](http://pub.dartlang.org/flutter)