add installation instructions to pkg packages

pkg/meta/lib/meta.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.
 
 /**
  * This library contains the definitions of annotations that provide additional
  * semantic information about the program being annotated. These annotations are
  * intended to be used by tools to provide a better user experience.
+ *
+ * ## Installing ##
+ *
+ * Use [pub][] to install this package. Add the following to your `pubspec.yaml`
+ * file.
+ *
+ *     dependencies:
+ *       meta: any
+ *
+ * And then run `pub install`.
+ *
+ * [pub]: http://pub.dartlang.org
  */
 library meta;
 
 /**
  * An annotation used to mark a class, field, getter, setter, method, top-level
  * variable, or top-level function as one that should no longer be used. Tools
  * can use this annotation to provide a warning on references to the marked
  * element.
  */
 const deprecated = const _Deprecated();
 
 class _Deprecated {
   const _Deprecated();
 }
 
 /**
  * An annotation used to mark an instance member (method, field, getter or
  * setter) as overriding an inherited class member. Tools can use this
  * annotation to provide a warning if there is no overridden member.
  */
 const override = const _Override();
 
 class _Override {
   const _Override();
 }