Added an introduction option by passing in a file with markdown.

pkg/docgen/bin/sdk-introduction.md

Index: pkg/docgen/bin/sdk-introduction.md
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+Welcome to the Dart API reference documentation,
+covering the official Dart APIs (dart:*)
+as well as other packages that live in the Dart project.
+For help using the Dart APIs, also see
+[www.dartlang.org](https://www.dartlang.org)
+pages such as the following:
+
+  * [Programmer's Guide](https://www.dartlang.org/docs/)
+  * [A Tour of the Dart Libraries](https://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html)
+  * [Dart Cookbook](https://www.dartlang.org/docs/cookbook/)
+  * [Articles](https://www.dartlang.org/articles/)
+  
+The 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
+[Guidelines for Dart Doc Comments](http://www.dartlang.org/articles/doc-comment-guidelines/),
+[Contributing](https://code.google.com/p/dart/wiki/Contributing),
+and, if you're contributing to DOM-related docs such as dart:html,
+[Contributing HTML Documentation](https://code.google.com/p/dart/wiki/ContributingHTMLDocumentation).