Add explanatory comment to empty BUILD file(s)

Change-Id: Ifc4e5146672ffa35ba45882cafaae82bf9ea5751
diff --git a/tools/bzl/BUILD b/tools/bzl/BUILD
index e69de29..c5ed0b7 100644
--- a/tools/bzl/BUILD
+++ b/tools/bzl/BUILD
@@ -0,0 +1 @@
+# Empty file required by Bazel