attendees should not use blackdown jdk under linux

docs/teaching/exercises/README.txt

@@ -23,6 +23,13 @@ Folder: aj-<event>:
     j2sdk for win, linux
     aspectj for everybody
 
+------------------------------
+Requirements for users:
+
+* We learned at OOPSLA 2004 that the system doesn't work well at
+  all under linux using the blackdown JDK.  Use Sun's jdk!
+
+
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