add installation instructions to pkg packages

pkg/http/lib/http.dart

 // Copyright (c) 2013, 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 composable, [Future]-based library for making HTTP requests.
 ///
+/// ## Installing ##
+///
+/// Use [pub][] to install this package. Add the following to your
+/// `pubspec.yaml` file.
+///
+///     dependencies:
+///       http: any
+///
+/// Then run `pub install`.
+///
+/// For more information, see the
+/// [http package on pub.dartlang.org](http://pub.dartlang.org/packages/http).
+///
 /// The easiest way to use this library is via the top-level functions. They
 /// allow you to make individual HTTP requests with minimal hassle:
 ///
 ///     import 'package:http/http.dart' as http;
 ///
 ///     var url = "http://example.com/whatsit/create";
 ///     http.post(url, fields: {"name": "doodle", "color": "blue"})
 ///         .then((response) {
 ///       print("Response status: ${response.statusCode}");
 ///       print("Response body: ${response.body}");
@@ skipping 26 matching lines @@
 ///       final String userAgent;
 ///       final http.Client _inner;
 ///
 ///       UserAgentClient(this.userAgent, this._inner);
 ///
 ///       Future<StreamedResponse> send(BaseRequest request) {
 ///         request.headers[HttpHeaders.USER_AGENT] = userAgent;
 ///         return _inner.send(request);
 ///       }
 ///     }
+///
+/// [pub]: http://pub.dartlang.org
 library http;
 
 import 'dart:async';
 import 'dart:typeddata';
 import 'dart:uri';
 
 import 'src/client.dart';
 import 'src/response.dart';
 
 export 'src/base_client.dart';
@@ skipping 101 matching lines @@
 /// For more fine-grained control over the request and response, use [Request]
 /// instead.
 Future<Uint8List> readBytes(url, {Map<String, String> headers}) =>
   _withClient((client) => client.readBytes(url, headers: headers));
 
 Future _withClient(Future fn(Client)) {
   var client = new Client();
   var future = fn(client);
   return future.whenComplete(client.close);
 }