Improve source autodiscovery failure message

The message we emitted previously did not provide any instructions on
how to resolve the issue, leaving users to guess what they need to do.

src/Psalm/Config/Creator.php

@@ -132,7 +132,7 @@ public static function getLevel(array $issues, int $counted_types): int
                 $issues_at_level[$issue_level][$issue_type] = 0;
             }
 
-            $issues_at_level[$issue_level][$issue_type] += 100/$counted_types;
+            $issues_at_level[$issue_level][$issue_type] += 100 / $counted_types;
         }
 
         foreach ($issues_at_level as $level => $issues) {
@@ -186,7 +186,10 @@ public static function getPaths(string $current_dir, ?string $suggested_dir): ar
 
             if (!file_exists($composer_json_location)) {
                 throw new ConfigCreationException(
-                    'Problem during config autodiscovery - could not find composer.json during initialization.'
+                    'Problem during source autodiscovery - could not find composer.json during initialization. '
+                    . 'If your project doesn\'t use Composer autoloader you will need to run '
+                    . '`psalm --init source_folder`, e.g. `psalm --init library` if your source files '
+                    . 'reside in `library` folder'
                 );
             }
             try {