Add comment to elucidate is_object() check

Author: sbesselsen

src/functions.php

@@ -8,6 +8,8 @@ function resolve($promiseOrValue = null)
         return $promiseOrValue;
     }
 
+    // Check is_object() first to avoid method_exists() triggering
+    // class autoloaders if $promiseOrValue is a string.
     if (is_object($promiseOrValue) && method_exists($promiseOrValue, 'then')) {
         $canceller = null;