Update README.md

Author: taku910

README.md

@@ -133,10 +133,11 @@ The following tools and libraries are required to build SentencePiece:
 * [cmake](https://cmake.org/)
 * C++11 compiler
 * [protobuf](https://github.com/google/protobuf) library
+* [gperftool](https://github.com/gperftools/gperftools) library (optional, 10-40% performance improvement can be obtained.)
 
 On Ubuntu, autotools can be installed with apt-get:
 ```
-% sudo apt-get install cmake pkg-config libprotobuf9v5 protobuf-compiler libprotobuf-dev
+% sudo apt-get install cmake pkg-config libprotobuf9v5 protobuf-compiler libprotobuf-dev libgoogle-perftools-dev 
 ```
 The name of the protobuf library is different between ubuntu distros. Please enter appropriate command for your Ubuntu version.