Adds referee screen and deities.

Updates bestiary with links to nonhuman racial modifiers.

src/pages/rules/basics.md

@@ -25,9 +25,14 @@ Sometimes, dice are rolled. These dice are described by how many sides they have
 
 Simple tasks are assumed to succeed. If the task is dangerous or complicated, roll the dice for a _check_.
 
-For most checks, roll a 20-sided die (d20) under a character's most relevant _ability_ score.
+For most checks, roll a 20-sided die (d20) at-or-over the difference of (20 - the character's most relevant _ability_ score).
 
-For hard (or very hard) tasks, you can subtract 1 (or 2) from the ability before the roll.
+-   For hard (or very hard) tasks, you can subtract 1 (or 2) from the roll before the die is thrown.
+
+Some results are special:
+
+-   A natural 20 represents a critical success: the check automatically succeeds, and often includes a bonus for the roller.
+-   A natural 1 is a critical failure: the check automatically fails, and the attempt often causes an additional complication or challenge for the roller.
 
 ### Abilities