Add troll hint

Shepherd.tex

@@ -286,7 +286,8 @@ them. However, you are free to do the challenges in any order you want.
 \begin{description}
     \item[Session Management Challenge 1] Try replacing \enquote{user} with
         \enquote{administrator}. But where?
-    \item[Poor Data Validation 1]
+    \item[Poor Data Validation 1]. The \enquote{troll} here means the third
+        image, i.e. a \enquote{trollface}. Google it if you are unsure.
     \item[Cross Site Scripting 1]
     \item[Session Management Challenge 2] Try attacking the password reset.
     \item[Session Management Challenge 3]