outputting friendly warning if build target contains commas

src/meta/build.js

@@ -143,6 +143,11 @@ function build(targets, callback) {
 			target = target.toLowerCase().replace(/-/g, '');
 			if (!aliases[target]) {
 				winston.warn('[build] Unknown target: ' + target);
+				if (target.indexOf(',') !== -1) {
+					winston.warn('[build] Are you specifying multiple targets? Separate them with spaces:');
+					winston.warn('[build]   e.g. `./nodebb build adminjs tpl`');
+				}
+
 				return false;
 			}