make the message more verbose

Scripts/developer_scripts/test_merge_of_branch

@@ -48,7 +48,7 @@ detect_wrong_permissions || exit 1
 echo '.. Testing licenses...'
 detect_packages_licenses || exit 1
 if ! ${git} diff --exit-code > /dev/null; then
-    echo '=> There are modifications in licenses:'
+    echo '=> There are modifications in licenses (check if all modified/added headers files contain a valid SPDX tag compatible with the one in package_info/PKG/license and that no license text is present):'
     ${git} status --porcelain
     exit 1
 fi