add installation instructions to pkg packages

pkg/stack_trace/lib/stack_trace.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.
 
+/**
+ * ## Installing ##
+ *
+ * Use [pub][] to install this package. Add the following to your `pubspec.yaml`
+ * file.
+ *
+ *     dependencies:
+ *       stack_trace: any
+ *
+ * And then run `pub install`.
+ *
+ * [pub]: http://pub.dartlang.org
+ */
 library stack_trace;
 
 export 'src/trace.dart';
 export 'src/frame.dart';