zulip_bots: Add doc.md for game_of_fifteen bot.

Finishes #513.

Author: anehls93

zulip_bots/zulip_bots/bots/game_of_fifteen/doc.md

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+# Game of Fifteen bot
+
+This bot is designed to let you play a [game of fifteen](https://en.wikipedia.org/wiki/15_puzzle).
+
+The goal of the game is to arrange all numbers from smallest to largest,
+starting with the grey question mark in the upper left corner, and then
+moving through each row till one reaches the end.
+
+## Usage
+
+To start a new game, simply type:
+
+```
+@game_of_fifteen
+```
+
+`move <tile1> <tile2>` - This command is used to pick which number to
+    switch with the grey question mark. Only pieces adjacent to the
+    grey question mark may be moved.