add explicit multiple format example (#2044)

* add explicit multiple format example

in reference to https://github.com/vhf/free-programming-books/pull/2023

* .pdf link wasn't html doh

CONTRIBUTING.md

@@ -88,3 +88,11 @@ GOOD: * [Another Awesome Book](http://example.com/book.pdf) (PDF)
 BAD : * [Another Awesome Book](http://example.com/book.pdf)- John Doe
 GOOD: * [Another Awesome Book](http://example.com/book.pdf) - John Doe (PDF)
 ```
+
+- Multiple formats:
+
+```
+BAD : * [Another Awesome Book](http://example.com/)- John Doe (HTML)
+BAD : * [Another Awesome Book](https://downloads.example.org/book.html)- John Doe (download site)
+GOOD: * [Another Awesome Book](http://example.com/) - John Doe (HTML) [(PDF, EPUB)](https://downloads.example.org/book.html)
+```