Add sample comment that demonstrates how to use SPDX-License-Identifier

[vbaderks]

Some projects on Github are starting to use SPDX-License_Identifiers to make it easy for the end-user to identify the license applied to that project / single file. As the cpp_start_project is a great project to copy and paste from, it could he helpful to use these identifiers. They typical look like:

// Copyright (c) <copyright owner>
// SPDX-License-Identifier: <license identifier>

and can be used in .cpp files, CMakeLists.txt files,.clang-format, etc.

A good example can be found in the https://github.com/microsoft/STL repository.