Add a trivial README for the mojo/ directory in the SDK.

packages/flutter/lib/mojo/README.md

+This directory contains code for exposing mojo services to Sky apps.
+For example, keyboard.dart wraps the mojo keyboard service in a more
+convenient Dart class.
+
+Files in this directory (and its subdirectories) only depend on core
+Dart libraries, `dart:sky`, `dart:sky.internals`, the 'mojo' package,
+the 'mojo_services' package, and `../base/*`.