Add contributors to attribution. (#5211)

LICENSE

@@ -3,5 +3,5 @@ Creative Commons Attribution 4.0 International License. To view a copy of
 this license, visit https://creativecommons.org/licenses/by/4.0/ or send
 a letter to Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.
 
-It is attributed to Victor Felder and the Free Ebook Foundation.
+It is attributed to Victor Felder, the Free Ebook Foundation, and contributors.
 https://github.com/EbookFoundation/free-programming-books