AToTB: Warn in the description how hard the challenging difficulty is

data/campaigns/Two_Brothers/_main.cfg

@@ -18,7 +18,9 @@
     {CAMPAIGN_DIFFICULTY EASY "units/human-loyalists/horseman/horseman.png~RC(magenta>red)" ( _ "Horseman") ( _ "Beginner")} {DEFAULT_DIFFICULTY}
     {CAMPAIGN_DIFFICULTY HARD "units/human-loyalists/grand-knight/grand-knight.png~RC(magenta>red)" ( _ "Grand Knight") ( _ "Challenging")}
 
-    description= _ "An evil mage is threatening the small village of Maghre and its inhabitants. The village’s mage sends to his warrior brother for help, but not all goes as planned. Can you help?
+    description= _ "An evil mage is threatening the small village of Maghre and its inhabitants. The village’s mage sends to his warrior brother for help, but not all goes as planned. Can you help?" + "
+
+<small>" + _ "This campaign has two difficulties, with a large difference between them. The Grand Knight difficulty is intended for players who’ve already completed Liberty and Heir to the Throne." + "</small>
 
 " + _"(Novice level, 4 scenarios.)"