Update third_party/protobuf to protobuf-v3.0.0-beta-3

third_party/protobuf/util/python/BUILD

Index: third_party/protobuf/util/python/BUILD
diff --git a/third_party/protobuf/util/python/BUILD b/third_party/protobuf/util/python/BUILD
index 358c381c592cf9f08df89053dc5c690b52953101..3ac03856690e43bba992ea8e5a1047e6435eed61 100644
--- a/third_party/protobuf/util/python/BUILD
+++ b/third_party/protobuf/util/python/BUILD
@@ -1,7 +1,17 @@
 # This is a placeholder for python headers. Projects needing to use
 # fast cpp protos in protobuf's python interface should build with
 # --define=use_fast_cpp_protos=true, and in addition, provide
-# //util/python:python_headers dependency that in turn provides Python.h.
+# //external:python_headers dependency that in turn provides Python.h.
+#
+# Projects that include protobuf using a Bazel external repository will need to
+# add a workspace rule to their WORKSPACE files to add an external workspace
+# that includes the Python headers. For example, the protobuf WORKSPACE file
+# includes the following local_repository rule that points to this directory:
+#
+# new_local_repository(
+#   name = "python",
+#   path = __workspace_dir__ + "/util/python",
+# )
 cc_library(
     name = "python_headers",
     visibility = ["//visibility:public"],