Adds CMake support.

[adishavit]

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[kylemcdonald]

thanks so much! i'm happy to merge this if you can also add a (very short, couple sentence) explanation to the readme about how to use cmake to build the project.

[adishavit]

CMake is a build generator. Using it depends on the platform you are targeting.
When there's a CMakeLists.txt file, you run CMake on it with the target build system and it generates the makefiles, MSVC solutions or whatever build system you need.
It is relatively easy to use, but since it is a "meta"-tool the actual usage depends on your target platform and target build system. I don't really know what to write... Having this file is usually enough to indicate CMake support.