Added a message to make it clearer to newcomers that you can leave early.

The game previously didn't give any clear indication that secondary
objectives are optional. This message should make this feature more
obvious.

src/missions.cpp

@@ -583,6 +583,11 @@ bool allMissionsCompleted()
 			}
 			else
 			{
+				if (currentMission.remainingObjectives2 > 0)
+				{
+					setInfoLine("Emergency warp drive activated. Press button to engage.", FONT_CYAN);
+				}
+
 				if (game.area != MISN_INTERCEPTION)
 					setInfoLine("*** All Primary Objectives Completed ***", FONT_GREEN);
 				else