add guiding for using self-prepared protobuf lib

README.md

@@ -84,6 +84,14 @@ On OSX, you can use brew:
 ```
 % brew install protobuf autoconf automake libtool
 ```
+Use your prepared protobuf library:
+```
+Setup below environment variables before build
+% export PROTOBUF=<path_to_protobuf>
+% export PROTOC="$PROTOBUF/bin/protoc"
+% export PROTOBUF_LIBS="-L$PROTOBUF/lib -lprotobuf -D_THREAD_SAFE"
+% export PROTOBUF_CFLAGS="-I$PROTOBUF/include -D_THREAD_SAFE" 
+```
 
 ## Build and Install SentencePiece
 ```