Add comment of grpc.tar.xz (#11153)

Author: gongweibao

cmake/external/grpc.cmake

@@ -45,6 +45,7 @@ ExternalProject_Add(
     #    checkout and clean other dirs under third_party
     # 4. remove .git, and package the directory.
     URL "http://paddlepaddledeps.bj.bcebos.com/grpc-v1.8.x.tar.gz"
+    URL_MD5  "c9c58ee7d0e8929a63155af6a2ecdbd0"
     PREFIX          ${GRPC_SOURCES_DIR}
     UPDATE_COMMAND  ""
     CONFIGURE_COMMAND ""