add a readme for the api docs

sdk/api_readme.md

+Welcome to the Dart API reference documentation,
+covering the official Dart API libraries.
+Some of the most fundamental Dart libraries include:
+   
+  * [dart:core](./dart_core/index.html):
+    Core functionality such as strings, numbers, collections, errors,
+    dates, and URIs.
+  * [dart:html](./dart_html/index.html):
+    DOM manipulation for web apps.
+  * [dart:io](./dart_io/index.html):
+    I/O for command-line apps.
+  
+Except for dart:core, you must import a library before you can use it.
+Here's an example of importing dart:html, dart:math, and a
+third popular library called [polymer.dart](https://www.dartlang.org/polymer-dart/):
+  
+    import 'dart:html';
+    import 'dart:math';
+    import 'package:polymer/polymer.dart';
+  
+Polymer.dart is an example of a library that isn't
+included in the Dart download,
+but is easy to get and update using the _pub package manager_.
+For information on finding, using, and publishing libraries (and more)
+with pub, see [pub.dartlang.org](https://pub.dartlang.org).
+  
+The main site for learning and using Dart is
+[www.dartlang.org](https://www.dartlang.org).
+Check out these pages:
+  
+  * [Dart homepage](https://www.dartlang.org)
+  * [Tutorials](https://www.dartlang.org/docs/tutorials/)
+  * [Programmer's Guide](https://www.dartlang.org/docs/)
+  * [Samples](https://www.dartlang.org/samples/)
+  * [A Tour of the Dart Libraries](https://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html)
+  
+This API reference is automatically generated from the source code in the
+[Dart project](https://code.google.com/p/dart/).
+If you'd like to contribute to this documentation, see
+[Contributing](https://code.google.com/p/dart/wiki/Contributing)
+and
+[Writing API Documentation](https://code.google.com/p/dart/wiki/WritingApiDocumentation).