add copyright notice

e9ae6f5c · kaashoek · f6831753 · e9ae6f5c
--- a/web/index.txt
+++ b/web/index.txt
@@ -335,4 +335,5 @@ Robert Morris (rtm@mit.edu)
 You can reach all of us at 6.828-staff@pdos.csail.mit.edu.
+xv6 and lecture notes are copyright &copy; 2006-present by Russ Cox,
+Frans Kaashoek, and Robert Morris.