project: Document top-level CMakeLists.txt requirement

CMake requires the top-level CMakeLists.txt to contain a direct call to
the project() command and will insert one if there is not.  Document
this requirement since some authors have tried to use include() to load
a file calling the project command.
Merge topic 'doc-project-top-level' into next

4a71168 project: Document top-level CMakeLists.txt requirement