webhooks/trello/doc: Require users to download Trello script.

We now have a standalone zulip_trello.py script that can be run
on Python2 and Python3 without having to install the `zulip`
package.

zerver/webhooks/trello/doc.md

@@ -1,7 +1,7 @@
 Get Zulip notifications from your Trello boards!
 
 !!! tip ""
-    Note that [the Zapier integration][1] is usually a simpler way to
+    Note that [Zapier][1] is usually a simpler way to
     integrate Trello with Zulip.
 
 [1]: ./zapier
@@ -10,8 +10,7 @@ Get Zulip notifications from your Trello boards!
 
 1. {!create-bot-construct-url-indented.md!}
 
-1. We will first collect three items: a **Board ID**, an **API Key**, and a
-   **User Token**.
+1. **Log in to Trello**, and collect the following three items:
 
     * **Board ID**: Go to your Trello board. The URL should look like
       `https://trello.com/b/<BOARD_ID>/<BOARD_NAME>`. Note down the
@@ -24,45 +23,32 @@ Get Zulip notifications from your Trello boards!
       **Developer API Keys**, click on the **Token** link. Click on **Allow**.
       Note down the token generated.
 
-1. Download and install our
-   [Python bindings and example scripts](/api/installation-instructions).
+1. Make sure you have a working copy of Python. If you're running macOS or
+   Linux, you very likely already do. If you're running Windows you may or
+   may not.  If you don't have Python, follow the installation instructions
+   [here](https://realpython.com/installing-python/). Note that you do not
+   need the latest version of Python; anything 2.7 or higher will do.
 
-    Trello supports outgoing webhooks, but unfortunately, doesn't have a UI
-    for configuring them. Instead, you have to use the Trello API to create
-    webhooks. We have provided a convenient script for doing this with the
-    Trello API in the Zulip Python bindings. You'll need to run this script
-    once on any machine (it doesn't have to be your Zulip server) to setup
-    the webhook; after you've done that, Trello will send the notifications
-    directly to your Zulip server.
+1. Download [zulip-trello.py][2]. `Ctr+s` or `Cmd+s` on that page should
+   work in most browsers.
 
-1. Open `/usr/local/share/zulip/integrations/trello/zulip_trello_config.py`
-   with your favorite editor and change the following lines to specify
-   the Trello **API Key**, **User Token**, and the webhook URL constructed
-   above:
+1. Run the `zulip-trello` script in a terminal, after replacing the
+   arguments with the values collected above.
 
     ```
-    TRELLO_API_KEY = "<Trello API key generated above>"
-    TRELLO_TOKEN = "<Trello User Token generated above>"
-    ZULIP_WEBHOOK_URL = "<URL constructed above>"
+    python zulip_trello.py --trello-board-name <trello_board_name> \
+                           --trello-board-id   <trello_board_id> \
+                           --trello-api-key  <trello_api_key> \
+                           --trello-token <trello_token> \
+                           --zulip-webhook-url <zulip_webhook_url>
     ```
 
-1. Go to the `/usr/local/share/zulip/integrations/trello/` directory
-   and run the `zulip_trello.py` script, like so:
+    The `zulip_trello.py` script only needs to be run once, and can be run
+    on any computer with python.
 
-    ```
-    python zulip_trello.py <trello_board_name> <trello_board_id>
-    ```
+1. You can delete `zulip_trello.py` from your computer if you'd like.
 
-    Replace `<trello_board_name` and `<trello_board_id>` with the Trello
-    board's name and **Board ID**, respectively.
-
-    The `zulip_trello.py` script needs to be run only once to register
-    the webhook, and can be run from any machine.
-
-!!! tip ""
-    To learn more, see [Trello's webhooks documentation][2].
-
-[2]: https://developers.trello.com/page/webhooks
+[2]: https://raw.githubusercontent.com/zulip/python-zulip-api/master/zulip/integrations/trello/zulip_trello.py
 
 {!congrats.md!}