Add mirror references back as historical note

Instead of having the mirror note in the YYYY/index.html files
completely removed it is back but as a historical note.

2004/README.md

@@ -13,6 +13,12 @@ yours is lacking, you may need to compile using `gcc` instead of your
 local compiler.
 
 
+### Historical note:
+
+The IOCCC has a website and now has a number of international mirrors.
+The primary website can be found at <https://www.ioccc.org/>.
+
+
 ## Remarks on some of the entries
 
 We believe you will be impressed with this year's winning entries.  The [Best of