Windows build instructions update

[memrex]

I'm referring to this page which is a bit outdated: https://github.com/mcallegari/qlcplus/w ... -Build-Qt5

Follow the instructions of that page otherwise install it using all the defaults (c:\msys64 or c:\msys32 for 32bit system), then open the MSYS2 shell and type:
update-core

"update-core" does not exist anymore. Instead, "pacman -Syu" should be used.

Now close the MSYS2 Shell. From now on we'll use the 'MinGW-w64 Win32 Shell'. So open it.

The shell is now called "MSYS2 MinGW 32-bit"

The make commands can be significantly accelerated when using parallel compilation, e.g. "make -j4"