[PATCH] kref: add link to original documentation to the kref documentation.

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Documentation/kref.txt

@@ -208,4 +208,9 @@ preferred as it is a little neater.
 
 Corey Minyard <minyard@acm.org>
 
-A lot of this was lifted from Greg KH's OLS presentation on krefs.
+A lot of this was lifted from Greg Kroah-Hartman's 2004 OLS paper and
+presentation on krefs, which can be found at:
+  http://www.kroah.com/linux/talks/ols_2004_kref_paper/Reprint-Kroah-Hartman-OLS2004.pdf
+and:
+  http://www.kroah.com/linux/talks/ols_2004_kref_talk/
+