Add CMakeLists.txt file about qr-code-generator

[nayuki]

Thank you for taking the time to introduce me to how CMake is used on Windows in practice.

[nayuki]

• Do you mean it is easy to build my sample applications? Those are just demos. The point of the library is to be included into a real application project. Nothing could be easier than simply adding the 6 source files to an existing project.
• Similar to pull request #14 (which came 2 weeks later from another author).

[sahil56]

First thing is that your demos will be built in a single step and exe file will be created for them.
Second thing is, there is no need to change existing files or project structure, only a single CMakeLists.txt needs to be added in c and cpp folders.
Thirdly, I think it is preferable to link to a library than including source files in the projects (even if there are just two or three).

Making use of Makefile on windows can be sometimes tough as some developers do not use autotools. It is always good for a project to provide both options of make and cmake. cmake is preferred in windows platform.