Added a missing line to readme file (#477)

Summary:
Going through the steps, I got a build error and looking into the problem I realised the submodules were not fetched.

Pull Request resolved: #477

Reviewed By: adshastri

Differential Revision: D43161233

Pulled By: haochenuw

fbshipit-source-id: c54f97617d813d97add04d8d8cf678f3810cd452

README.md

@@ -182,6 +182,7 @@ sudo apt-get install -y libcurl4-openssl-dev libssl-dev uuid-dev zlib1g-dev libp
 # TODO: add version control
 git clone https://github.com/aws/aws-sdk-cpp.git
 cd aws-sdk-cpp || exit
+git submodule update --init --recursive
 mkdir build
 cd build || exit
 # -DCUSTOM_MEMORY_MANAGEMENT=0 is added to avoid Aws::String and std::string issue