Treat empty target as a warning (likely a mistake, e.g. #437).

Author: nedtwigg

plugin-maven/src/main/java/com/diffplug/spotless/maven/SpotlessApplyMojo.java

@@ -70,8 +70,7 @@ protected void process(Iterable<File> files, Formatter formatter, UpToDateChecke
 			getLog().info(String.format("Spotless.%s is keeping %s files clean - %s were changed to be clean, %s were already clean, %s were skipped because caching determined they were already clean",
 					formatter.getName(), totalProcessed, cleaned, checkedButAlreadyClean, skippedAsCleanCache));
 		} else {
-			getLog().debug(String.format("Spotless.%s is not considering a single file",
-					formatter.getName()));
+			getLog().warn(String.format("Spotless.%s has no target files. Examine your `<includes>`: https://github.com/diffplug/spotless/tree/main/plugin-maven#quickstart", formatter.getName()));
 		}
 	}
 }