Added fixnum package docs.

pkg/fixnum/README.md

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+fixnum
+======
+
+A fixed-size integer library for Dart.
+- - -
+The fixnum package provides data types for signed 32- and 64-bit integers.
+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).