Add year link to title in 2004/README.md

That is link to years.html#2004. This seems like a worthy thing to do
and I will do it for the rest of the years too as I edit the year
README.md files. I probably (:-) ) won't bother repeating this comment
to save time and length of git log.

2004/README.md

@@ -1,4 +1,4 @@
-# 2004 marked the "The Seventeenth International Obfuscated C Code Contest"
+# [2004](/years.html#2004) marked the "The Seventeenth International Obfuscated C Code Contest"
 
 
 ## Standard IOCCC stuff