add installation instructions to pkg packages

pkg/args/lib/args.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.
 
 /**
  * This library lets you define parsers for parsing raw command-line arguments
  * into a set of options and values using [GNU][] and [POSIX][] style options.
  *
+ * ## Installing ##
+ *
+ * Use [pub][] to install this package. Add the following to your `pubspec.yaml`
+ * file.
+ *
+ *     dependencies:
+ *       args: any
+ *
+ * Then run `pub install`.
+ *
+ * For more information, see the
+ * [args package on pub.dartlang.org](http://pub.dartlang.org/packages/args).
+ *
  * ## Defining options ##
  *
  * To use this library, you create an [ArgParser] object which will contain
  * the set of options you support:
  *
  *     var parser = new ArgParser();
  *
  * Then you define a set of options on that parser using [addOption()] and
  * [addFlag()]. The minimal way to create an option is:
  *
@@ skipping 183 matching lines @@
  *           [arm]       ARM Holding 32-bit chip
  *           [ia32]      Intel x86
  *
  * To assist the formatting of the usage help, single line help text will
  * be followed by a single new line. Options with multi-line help text
  * will be followed by two new lines. This provides spatial diversity between
  * options.
  *
  * [posix]: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap12.html#tag_12_02
  * [gnu]: http://www.gnu.org/prep/standards/standards.html#Command_002dLine-Interfaces
+ * [pub]: http://pub.dartlang.org
  */
 library args;
 
 import 'src/parser.dart';
 import 'src/usage.dart';
 
 /**
  * A class for taking a list of raw command line arguments and parsing out
  * options and flags from them.
  */
@@ skipping 192 matching lines @@
           'Could not find an option named "$name".');
     }
 
     return _options[name];
   }
 
   /** Get the names of the options as an [Iterable]. */
   Iterable<String> get options => _options.keys;
 }