Documentation on how to build CMake header only libb

[annell]

I think you could actually clarify a little on how to link up against this library in your wiki page.
Have a look at how I did for https://github.com/annell/physim-cpp

Essentially:

1. Clone the subrepo
2. Add the subdirectory with add_directory()
3. Link up against your math library using target_link_library(.... SFMLMath)
4. Include it where you want to use it in your project using #include <SFMLMath.h>

Sorry for being picky... 😅 But it's a shame when you have all the pieces there to not share how to include it correctly!