Add blurb about page to add/delete your own SFS public keys.

www/doc/sfs.html

@@ -66,3 +66,13 @@ To rex into <tt>users.emulab.net</tt>:
 	<code><pre>
 	sfsagent
 	rex -x /sfs/netbed/users.emulab.net		</code></pre>
+
+<p>
+As with SSH keys, you may also upload the public parts of your own SFS
+keys. We store those public keys in our database, and distribute them
+to all of the nodes in your experiments. This is better and safer than
+copying the Emulab generated key back to your home machine, since
+generally your own keys are passphrase protected and secure from theft
+when on a (semi)public network. You may add and delete SFS public keys
+from your user profile by going to the <a href=../moduserinfo.php3>
+Update User Information</a> page.