docs: Add reference to zulip-bot-output script.

docs/bots-guide.md

@@ -68,6 +68,9 @@ server.  It assumes you want to use one of the existing `zulip_bots/bots`
 bots in your Zulip organization.  If you want to write a new one, you
 just need to write the `<my-bot>.py` script and put it into `zulip_bots/bots/<my-bot>` directory.
 
+*Looking for an easy way to test a bot's output? Check out [this](
+ #testing-a-bot-s-output) section.*
+
 You need:
 
 * An account in an organization on a Zulip server
@@ -143,6 +146,26 @@ You need:
   the `helloworld` bot, you should expect the bot to respond with
   "beep boop".
 
+### Testing a bot's output
+
+If you just want to see how a bot reacts to a message, but don't want to set it up on a server,
+we have a little tool to help you out: `zulip-bot-output`
+
+* Follow the first step in [How to run a bot](#how-to-run-a-bot) to install all requirements.
+
+* Run `zulip-bot-output <bot-name> --message "<your-message>"` to test one of the bots in
+  [`zulip_bots/bots`](https://github.com/zulip/python-zulip-api/tree/master/zulip_bots/zulip_bots/bots)
+
+  * Example: `zulip-bot-output converter --message "12 meter yard"`
+
+    Response: `12.0 meter = 13.12336 yard`
+
+* Run `zulip-bot-output <path/to/bot.py> --message "<your-message>"` to specify the bot's path yourself.
+
+  * Example: `zulip-bot-output zulip_bots/zulip_bots/bots/converter/converter.py --message "12 meter yard"`
+
+    Response: `12.0 meter = 13.12336 yard`
+
 ## Zulip Botserver
 
 The Zulip Botserver is for people who want to