While at it, add comment to indicate what the version signifies.

[a3f: updated commit message]

So user code can use add_subdirectory to build it (similar to what we do
with GLFW or what the projects/CMake/CMakeLists.txt can do).

The CMakeLists.txt checks for an installed raylib and downloads and
installs one if none is found. Afterwards, it builds core_basic_window.c