Update FAQ

docs/faq.md

@@ -55,3 +55,7 @@ file in Excel, the actual width is 0.71 less than it should be.
 The short answer is that PhpSpreadsheet uses a measure where padding is
 included. See [how to set a column's width](./topics/recipes.md#setting-a-columns-width)
 for more details.
+
+## I cannot serialize or json_encode my Spreadsheet
+
+No, you can't. Consider the Spreadsheet object as a PHP resource, which cannot be serialized.