Allow to use custom txt file for macOS installer

utils/package-osx-bundles.sh

@@ -44,7 +44,10 @@ cd ..
 
 DPF_UTILS_DIR=$(dirname ${0})
 
-sed -e "s|@name@|${NAME}|" ${DPF_UTILS_DIR}/plugin.pkg/welcome.txt.in > build/welcome.txt
+# can be overridden by environment variables
+WELCOME_TXT=${WELCOME_TXT:=${DPF_UTILS_DIR}/plugin.pkg/welcome.txt.in}
+
+sed -e "s|@name@|${NAME}|" "${WELCOME_TXT}" > build/welcome.txt
 sed -e "s|@builddir@|${PWD}/build|" \
     -e "s|@lv2bundleref@|dpf-${SNAME}-lv2bundles.pkg|" \
     -e "s|@vst2bundleref@|dpf-${SNAME}-vst2bundles.pkg|" \