Add help for Balloon.

user/doc/balloon.md

@@ -1,5 +1,11 @@
-# Balloon
+# Balloon Travellers
 
 (Press Enter to view key bindings.)
 
-Balloon help.
+During the balloon trip, you need to end your turn at five destinations marked with question marks (?) to beat the game. You have at most 24 turns. You lose the game when running out of turns. Reaching a destination restores 10 turns for once.
+
+Every turn the balloon moves two steps. Firstly, it goes with the wind. The wind direction is indicated by an arrow and it changes every three turns. [1] Secondly, the balloon responds to your key input, which could be one of four directions or waiting.
+
+You can move over the border of the dungeon and reappear on the opposite side. When there is an obstacle (#) in the destination, the balloon will try to step back one grid if possible.
+
+[1] The wind direction is more likely to change into its vertical counterparts: from left/right to up/down, or vice versa. And it is less likely to remain unchanged for another three turns. It never changes to the opposite direction: between left and right, or up and down.