Export LICENSE

This makes it easier to include the license in redistributions, as required by the license.

BUILD.bazel

@@ -36,6 +36,8 @@ package(default_visibility = ["//visibility:public"])
 
 licenses(["notice"])
 
+exports_files(["LICENSE"])
+
 config_setting(
     name = "windows",
     constraint_values = ["@bazel_tools//platforms:windows"],