Add comments.

cmake/external/protobuf.cmake

@@ -191,6 +191,8 @@ FUNCTION(build_protobuf TARGET_NAME BUILD_FOR_HOST)
     SET(PROTOBUF_REPO "https://github.com/google/protobuf.git")
     SET(PROTOBUF_TAG "9f75c5aa851cd877fb0d93ccc31b8567a6706546")
     IF(MOBILE_INFERENCE)
+        # The reason why the official version is not used is described in
+        # https://github.com/PaddlePaddle/Paddle/issues/6114
         SET(PROTOBUF_REPO "https://github.com/qingqing01/protobuf.git")
         SET(PROTOBUF_TAG "v3.2.0")
         IF(NOT BUILD_FOR_HOST)