Add example cmake file for win32 directx11 backend

[jalius]

I want to add a CMakeLists.txt file to the examples folder. Currently there is only a Visual Studio build script there.

This is a very minimal CMake configuration to build the example exe. I think it provides a good starting point for new people.

I tested it with a minimal installation of MSYS2, CMake, Ninja, and GCC. It should also work with other generators.

Let me know any thoughts.

Steps to build:
$ cd example_win32_directx11
$ mkdir build && cd build
$ cmake .. (Generator flag optional: eg -G 'MSYS Makefiles')
$ make OR ninja OR .sln (depends on your cmake-generator)

[jalius]

P.S. I have been using this project for a few months and I really like it. That's why I'm here.