Create a top-level BUILD file that exports the LICENSE file.

This is typically used in other bazel projects (e.g. tensorflow) that uses absl-py as third party packages.

Using `BUILD.bazel` to avoid potential conflicts with a `build` directory in case insensitive file systems.

Context: #160 (comment)
PiperOrigin-RevId: 399963516
Change-Id: Icdac6efeff0a83c263b3d449ca1f18c24879af72

BUILD.bazel

@@ -0,0 +1,21 @@
+# Copyright 2021 The Abseil Authors.
+#
+# 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.
+
+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])
+
+exports_files([
+    "LICENSE",
+])