Add help for desert.

user/doc/desert.md

@@ -1,3 +1,32 @@
 # Desert
 
-Desert help. Sandworm.
+You need to bump and collect 5 quality spices (*) from a living sandworm to beat the game. But you cannot press Space to wait in desert, which makes the collecting job a little bit harder.
+
+A sandworm has a head (diamond), a tail (-) and body segments (+). The third, fourth and fifth segment contains spice (?) or quality spice (*). You can only interact with segments that have spices. Other sandworm parts (head, body and tail) act like indestructible walls.
+
+Every turn a sandworm moves one grid in a random direction. Eventually it will tunnel into ground and leave a wall of dune behind. An immobilized sandworm or one with fewer than three spices are more eager to go underground.
+
+A dunes is composed of sand walls (#) and spices (?). You can bump and destroy a wall block or collect a spice just as from a sandworm. You do not move when bumping. It can be used as poor man's waiting key.
+
+You might lose the game due to one of three reasons.
+
+You can survive in desert for at most 24 turns. Every spice and quality spice extends your remaining time by 4 turns.
+
+PC's symbol changes into a rectangular when being adjacent to a sandworm's head. The game ends if the sandworm intends to move into PC's position the next turn.
+
+When your turn starts, if you are surrounded by four solid blocks, either press Space or bump into a sandworm segment to end the game.
+
+    . # .   North (O): Sand wall.
+    . @ +   East (X): Sandworm segment.
+    + + +   South (X): Sandworm segment.
+            West (O): Floor.
+
+    % ? .   North (O): Spice.
+    % @ *   East (O): Quality spice.
+    % - +   South (X): Sandworm tail.
+            West (X): Edge of the dungeon.
+
+    % s +   North (X): Sandworm head.
+    % @ +   East (X): Sandworm segment.
+    % - +   South (X): Sandworm tail.
+            West (X): Edge of the dungeon.