Update faq.md

docs/faq.md

@@ -17,4 +17,6 @@ I'm also grateful for your donations:
 
 <script data-gratipay-username="wenzhixin" data-gratipay-widget="button" src="//gttp.co/v1.js"></script>
 
-Thank you!
+[![Donate](https://www.paypalobjects.com/en_US/i/btn/btn_donateCC_LG.gif)](https://www.paypal.com/cgi-bin/webscr?cmd=_xclick&business=wenzhixin2010%40gmail%2ecom&lc=C2&button_subtype=services&currency_code=USD&bn=PP%2dBuyNowBF%3abtn_donateCC_LG%2egif%3aNonHosted)
+
+Thank you!