pkg/js: prepare for 0.6.0 release

pkg/js/README.md

 Methods and annotations to specify interoperability with JavaScript APIs.
 
-*Note: This package is beta software.*
+*This packages requires Dart SDK 1.13.0.*
 
-*Note: This packages requires Dart SDK `>=1.13.0-dev.7`.*
+*This is beta software. Please files [issues].*
 
 ### Adding the dependency
 
 Add the following to your `pubspec.yaml`:
 
 ```yaml
 dependencies:
-  js: ^0.6.0-beta
+  js: ^0.6.0
 ```
 
-#### Passing functions to JavaScript.
-
-If you are passing a Dart function to a JavaScript API, you must wrap it using
-`allowInterop` or `allowInteropCaptureThis`.
-
-### Examples
+### Example
 
 See the [Chart.js Dart API](https://github.com/google/chartjs.dart/) for an
 end-to-end example.
 
+### Usage
+
 #### Calling methods
 
 ```dart
 // Calls invoke JavaScript `JSON.stringify(obj)`.
 @JS("JSON.stringify")
 external String stringify(obj);
 ```
 
 #### Classes and Namespaces
 
@@ skipping 28 matching lines @@
 ```js
 // JavaScript
 printOptions({responsive: true});
 ```
 
 If you want to use `printOptions` from Dart, you cannot simply pass a Dart `Map`
 object – they are are "opaque" in JavaScript.
 
 
 Instead, create a Dart class with both the `@JS()` and
 `@anonymous` annotations.

[Siggi Cherem (dart-lang), 2015/11/18 23:30:42]

mmm. not sure how to reduce the attention on @anonymous here like we talked
about. 

The main issue is that the code has to be visibly reachable to dart2js,
otherwise you'll see a crash. 

 
 ```dart
 // Dart
 void main() {
   printOptions(new Options(responsive: true));
 }
 
 @JS()
 external printOptions(Options options);
 
 @JS()
 @anonymous
 class Options {
   external bool get responsive;
 
   external factory Options({bool responsive});
 }
 ```
 
+#### Passing functions to JavaScript.
+
+If you are passing a Dart function to a JavaScript API, you must wrap it using
+`allowInterop` or `allowInteropCaptureThis`.
+
 ## Contributing and Filing Bugs
 
-Please file bugs and features requests on the [Github issue tracker](https://github.com/dart-lang/sdk/issues).
+Please file bugs and features requests on the [Github issue tracker][issues].
 
 We also love and accept community contributions, from API suggestions to pull requests.
 Please file an issue before beginning work so we can discuss the design and implementation.
 We are trying to create issues for all current and future work, so if something there intrigues you (or you need it!) join in on the discussion.
 
 Code contributors must sign the
 [Google Individual Contributor License Agreement](https://developers.google.com/open-source/cla/individual?csw=1).
+
+[issues]: https://goo.gl/j3rzs0