Use a prebuilt Stardoc release binary (#172)

Instead of forcing users to depend on @io_bazel so that Stardoc
is built from source for each invocation, a prebuilt Stardoc binary will be included
in this repository, and updated on release cuts.

This also eliminates buggy behavior when Stardoc and @io_bazel repositories
are out of sync in a user's workspace.

Closes #170
diff --git a/update-release-binary.sh b/update-release-binary.sh
new file mode 100755
index 0000000..a61fa46
--- /dev/null
+++ b/update-release-binary.sh
@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+# Copyright 2019 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# Renerates the Stardoc release binary from source in @io_bazel.
+#
+# This should only need to be run for cutting a new Stardoc release.
+
+set -eu
+
+echo "** Building Stardoc from source..."
+bazel build @io_bazel//src/main/java/com/google/devtools/build/skydoc:skydoc_deploy.jar
+
+echo "** Copying Stardoc binary..."
+cp bazel-bin/external/io_bazel/src/main/java/com/google/devtools/build/skydoc/skydoc_deploy.jar \
+    stardoc/stardoc_binary.jar
+
+echo "** Stardoc copied."