add_subdirectory given source "nlohmann_json" which is not an existing directory.

[hygxy]

I want to use this repository in "Embedded" mode using CMake. I did exactly what is described in README(place the entire source tree in a subdirectory and call add_subdirectory() in your CMakeLists.txt file), but get the following error:

"add_subdirectory given source "nlohmann_json" which is not an existing directory.“ 

Besides, I would like to know what does "..." means in line add_library(foo ...)

[nlohmann]

See https://github.com/nlohmann/json/tree/develop/test/cmake_add_subdirectory/project for an example.