Update Apple Silicon build instructions

Author: ann0see

COMPILING.md

@@ -119,7 +119,8 @@ brew link Qt@5 --force
 
 ### Generate Xcode Project file
 
-`qmake -spec macx-xcode Jamulus.pro`
+`qmake QMAKE_APPLE_DEVICE_ARCHS=arm64 QT_ARCH=arm64 -spec macx-xcode Jamulus.pro`
+**Note: if you still build on x86_64, not Apple silicon, you must replace `arm64` with `x86_64`.
 
 ### Print build targets and configuration in console