Update CONTRIBUTING.md to have an option to keep name anonymous if requested

Same changes as 1d792e6bbaf1463c152c975c32a4edc671c94ea8, which didn't
update the CONTRIBUTING.md

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

CONTRIBUTING.md

@@ -27,10 +27,10 @@ issue, please bring it to their attention right away!
 Please **DO NOT** file a public issue, instead send your report privately to
 [security@docker.com](mailto:security@docker.com).
 
-Security reports are greatly appreciated and we will publicly thank you for it.
-We also like to send gifts—if you're into schwag, make sure to let
-us know. We currently do not offer a paid security bounty program, but are not
-ruling it out in the future.
+Security reports are greatly appreciated and we will publicly thank you for it,
+although we keep your name confidential if you request it. We also like to send
+gifts—if you're into schwag, make sure to let us know. We currently do not
+offer a paid security bounty program, but are not ruling it out in the future.
 
 
 ## Reporting other issues