interactive bots: Create xkcd bot.

contrib_bots/lib/xkcd/docs.md

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+# xkcd bot
+
+xkcd bot is a Zulip bot that can fetch a comic strip from xkcd. To use xkcd
+bot you can simply call it with `@xkcd` followed by a command. Like this:
+
+```
+@xkcd <command>
+```
+
+xkcd bot has four commands:  
+
+1. `help`  
+This command is used to list all commands that can be used with this bot.
+You can use this command by typing `@xkcd help` in a stream.  
+![](xkcd-help.png)
+
+2. `latest`  
+This command is used to fetch the latest comic strip from xkcd. You can use
+this command by typing `@xkcd latest` in a stream.  
+![](xkcd-latest.png)
+
+3. `random`  
+This command is used to fetch a random comic strip from xkcd. You can use
+this command by typing `@xkcd random` in a stream, xkcd bot will post a
+random xkcd comic strip.  
+![](xkcd-random.png)
+
+4. `<comic_id>`  
+To fetch a comic strip based on id, you can directly use `@xkcd <comic_id>`,
+for example if you want to fetch a comic strip with id 1234, you can type
+`@xkcd 1234`, xkcd bot will post a comic strip with id 1234.  
+![](xkcd-specific-id.png)  
+
+If you type a wrong command to xkcd bot, xkcd bot will post information
+you'd get from `@xkcd help`.  
+![](xkcd-wrong-command.png)
+
+And if you type a wrong id, xkcd bot will post a message that an xkcd comic
+strip with that id is not available.  
+![](xkcd-wrong-id.png)