Restructure YAML package suitable for pub lish

pkg/yaml/README.md

+A parser for [YAML](http://www.yaml.org/).
+
+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));
+    }
+
+The source code for this package is at <http://code.google.com/p/dart>.
+Please file issues at <http://dartbug.com>. Other questions or comments can be
+directed to the Dart mailing list at <mailto:misc@dartlang.org>.