add installation instructions to pkg packages

pkg/stack_trace/lib/stack_trace.dart

Index: pkg/stack_trace/lib/stack_trace.dart
diff --git a/pkg/stack_trace/lib/stack_trace.dart b/pkg/stack_trace/lib/stack_trace.dart
index 5747b85b97d7bbb84cd4d7d704dd8925380865fb..39a8ccd1cdcf3eab1c0d3f669c9cb47b87a638f3 100644
--- a/pkg/stack_trace/lib/stack_trace.dart
+++ b/pkg/stack_trace/lib/stack_trace.dart
@@ -2,6 +2,19 @@
 // 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';