Add support for Visual Studio 16 2019

[treyyoder]

Requesting and update to support VS2019.

[xmrig]

Please explain your request a little more, VS2019 is already supported. Thank you.

[treyyoder]

The documentation only provides directions for VS2017.

Fumbling around with a few different cmake commands gives me the same result:

R:\DEV\community\xmrig-amd\build (master -> origin)
λ cmake .. -G "Visual Studio 16 2019" -A WIN64
CMake Error: Error: generator : Visual Studio 16 2019
Does not match the generator used previously: Visual Studio 15 2017 Win64
Either remove the CMakeCache.txt file and CMakeFiles directory or choose a different binary directory.

So I guess what I'm requesting is an update to the docs.