No cmake module finding benchmark when turning on the BUILD_TESTING option

[曹立]

Hi there,

I'm a new one on github. I tried to build the project with the BUILD_TESTING option ON with cmake and 

it didn't work because it didn't find the benchmark package. I checked the CMakeLists.txt, it called 

find_package(benchmark) and there isn't a cmake module to find such package. And I think there is no 

such native cmake module. So shall we add a Findbenchmark.cmake under cmake/module path? And 

the getting started part of readme doc didn't mention about benchmark, I think it should also be installed 

in advance. So maybe we should modify the readme doc?

Thank you!

Irving