webhooks/trello/doc.md: Add context for why we need to use a script.

zerver/webhooks/trello/doc.md

@@ -1,4 +1,10 @@
-### Get Zulip notifications from your Trello boards!
+Get Zulip notifications from your Trello boards!
+
+!!! tip ""
+    Note that [the Zapier integration][1] is usually a simpler way to
+    integrate Trello with Zulip.
+
+[1]: ./zapier
 
 1. {!create-stream.md!}
 
@@ -21,6 +27,14 @@
 1. Download and install our
    [Python bindings and example scripts](/api/installation-instructions).
 
+    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. 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
@@ -39,16 +53,16 @@
     python zulip_trello.py <trello_board_name> <trello_board_id>
     ```
 
-    Replace `<trello_board_name` and `<trello_board_id` with the Trello
+    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][1].
+    To learn more, see [Trello's webhooks documentation][2].
 
-[1]: https://developers.trello.com/page/webhooks
+[2]: https://developers.trello.com/page/webhooks
 
 {!congrats.md!}