Added fixnum package docs.

pkg/fixnum/lib/fixnum.dart

 // Copyright (c) 2012, 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.
 
+/**
+ * Signed 32- and 64-bit integer support.
+ *
+ * The integer implementations in this library are designed to work
+ * identically whether executed on the Dart VM or compiled to JavaScript.
+ *
+ * ## Installing
+ *
+ * Use [pub](http://pub.dartlang.org) to install this package. Add the
+ * following to your `pubspec.yaml` file:
+ *
+ *     dependencies:
+ *       fixnum: any
+ *
+ * Then run `pub install`.
+ *
+ * For more information, see the
+ * [fixnum package on pub.dartlang.org]
+ * (http://pub.dartlang.org/packages/fixnum).
+ */
 library fixnum;
 
 part 'src/intx.dart';
 part 'src/int32.dart';
 part 'src/int64.dart';