"pacman -Syu" needed for msys2 to build properly, but not reflected in README

When using the out-of-the-box msys2 to build, it complains that libfluidsynth-2.dll cannot be found. This is because msys2 ships with an 1.x version of fluidsynth (and thus only has libfluidsynth-1.dll), so we need to upgrade it to 2.x, which is the version used in Purr Data.