Update C++ README
Update C++ README

Merge pull request #548 from awsdocs/cpp_readme
awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples