readme: MSVC 2017 is the supported one now
readme: MSVC 2017 is the supported one now

also add note about pre-built master branch

zig-lang/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,zig-lang/zig,andrewrk/zig