add documentation details on the parameter `cmake_packages` of `build_module`

[rambausek]

Please excuse that this issue exists. I misinterpreted the parameter cmake_packages; I did not know that these packages have to provide a USE_FILE. The original proposal reported was a result of this misinterpretation, so I changed it to this stub.

[rambausek]

Instead of trying to include cmake modules of external libs, e.g. Qt5 it is probably better to write a small cmake package with a "USE_FILE" to be used by build_module (given via cmake_packages["MyPackage"])