Update readme

Author: Bill-hbrhbr

README.md

@@ -5,6 +5,7 @@ An open-source C++ library developed and used at YScope.
 # Usage
 Clone `ystdlib-cpp` into your project. Then, in your project's `CMakeLists.txt`, add the following:
 ```cmake
+set(YSTDLIB_CPP_BUILD_TESTING OFF)
 add_subdirectory(/path/to/ystdlib-cpp EXCLUDE_FROM_ALL)
 target_link_libraries(<target_name> <link_options>
     ystdlib::<lib_1> ystdlib::<lib_2> ... ystdlib::<lib_N>