add installation instructions to pkg packages

pkg/yaml/lib/yaml.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// A parser for [YAML](http://www.yaml.org/).
 ///
+/// ## Installing ##
+///
+/// Use [pub][] to install this package. Add the following to your
+/// `pubspec.yaml` file.
+///
+///     dependencies:
+///       yaml: any
+///
+/// Then run `pub install`.
+///
+/// For more information, see the
+/// [yaml package on pub.dartlang.org][pkg].
+///
+/// ## Using ##
+///
 /// Use [loadYaml] to load a single document, or [loadYamlStream] to load a
 /// stream of documents. For example:
 ///
 ///     import 'package:yaml/yaml.dart';
 ///     main() {
 ///       var doc = loadYaml("YAML: YAML Ain't Markup Language");
 ///       print(doc['YAML']);
 ///     }
 ///
 /// This library currently doesn't support dumping to YAML. You should use
 /// `stringify` from `dart:json` instead:
 ///
 ///     import 'dart:json' as json;
 ///     import 'package:yaml/yaml.dart';
 ///     main() {
 ///       var doc = loadYaml("YAML: YAML Ain't Markup Language");
 ///       print(json.stringify(doc));
 ///     }
+///
+/// [pub]: http://pub.dartlang.org
+/// [pkg]: http://pub.dartlang.org/packages/yaml
 library yaml;
 
 import 'src/composer.dart';
 import 'src/constructor.dart';
 import 'src/parser.dart';
 import 'src/yaml_exception.dart';
 
 export 'src/yaml_exception.dart';
 export 'src/yaml_map.dart';
 
@@ skipping 18 matching lines @@
 /// The return value is mostly normal Dart objects. However, since YAML mappings
 /// support some key types that the default Dart map implementation doesn't
 /// (null, NaN, booleans, lists, and maps), all maps in the returned document
 /// are [YamlMap]s. These have a few small behavioral differences from the
 /// default Map implementation; for details, see the [YamlMap] class.
 List loadYamlStream(String yaml) {
   return new Parser(yaml).l_yamlStream()
       .map((doc) => new Constructor(new Composer(doc).compose()).construct())
       .toList();
 }