link to ssd research paper

tsa/howto/retire-a-host.mdwn

@@ -110,6 +110,10 @@ This will take a long time. Note that it will start a GUI which is
 useful because it will give you timing estimates, which the
 commandline version [does not provide](https://github.com/martijnvanbrummelen/nwipe/issues/196).
 
+WARNING: this procedure doesn't cover the case where the disk is an
+SSD. See [this paper](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.187.3062&rep=rep1&type=pdf) for details on how classic data scrubbing
+software might not work for SSDs.
+
 When you return:
 
  1. start a `screen` session with a static `busybox` as your `SHELL`