add installation instructions to pkg packages

pkg/oauth2/lib/oauth2.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 client library for authenticating with a remote service via OAuth2 on
 /// behalf of a user, and making authorized HTTP requests with the user's OAuth2
 /// credentials.
 ///
+/// ## Installing ##
+///
+/// Use [pub][] to install this package. Add the following to your
+/// `pubspec.yaml` file.
+///
+///     dependencies:
+///       oauth2: any
+///
+/// Then run `pub install`.
+///
+/// For more information, see the
+/// [oauth2 package on pub.dartlang.org][pkg].
+///
 /// OAuth2 allows a client (the program using this library) to access and
 /// manipulate a resource that's owned by a resource owner (the end user) and
 /// lives on a remote server. The client directs the resource owner to an
 /// authorization server (usually but not always the same as the server that
 /// hosts the resource), where the resource owner tells the authorization server
 /// to give the client an access token. This token serves as proof that the
 /// client has permission to access resources on behalf of the resource owner.
 ///
 /// OAuth2 provides several different methods for the client to obtain
 /// authorization. At the time of writing, this library only supports the
@@ skipping 74 matching lines @@
 ///           .then((result) {
 ///         // Once we're done with the client, save the credentials file. This
 ///         // ensures that if the credentials were automatically refreshed
 ///         // while using the client, the new credentials are available for the
 ///         // next run of the program.
 ///         return credentialsFile.open(FileMode.WRITE).then((file) {
 ///           return file.writeString(client.credentials.toJson());
 ///         }).then((file) => file.close()).then((_) => result);
 ///       });
 ///     }).then(print);
+///
+/// [pub]: http://pub.dartlang.org
+/// [pkg]: http://pub.dartlang.org/packages/oauth2
 library oauth2;
 
 export 'src/authorization_code_grant.dart';
 export 'src/client.dart';
 export 'src/credentials.dart';
 export 'src/authorization_exception.dart';
 export 'src/expiration_exception.dart';