integrations/trello: Stop using config files in the Trello script.

A few users have complained about how hard it is to find the
Trello script on their systems after installing the `zulip`
package. One way to solve this issue is to make the Trello
script a part of the exported console scripts in the zulip
package, which would mean that the user would not have to
navigate to a particular directory in order to find the script,
but could run it from a terminal directly.

However, to make this happen, we need to minimize the script's
reliance on external configuration files, because we don't want
the user to have to figure out where the config file lives.

zulip/integrations/trello/README.md

@@ -2,12 +2,22 @@
 
 Usage :
 
-1. Fill the needed information in `zulip_trello_config.py` :
+1. Make sure you have all of the relevant Trello credentials before
+   executing the script:
 
-    - The Trello API KEY,
-    - The Trello TOKEN,
+    - The Trello API KEY
+    - The Trello TOKEN
     - The Zulip webhook URL
+    - Trello board name
+    - Trello board ID
 
 2. Execute the script :
 
-    $ python zulip_trello.py <trello_board_name> <trello_board_id>
+    $ 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>
+
+For more information, please see Zulip's documentation on how to set up
+a Trello integration [here](https://zulipchat.com/integrations/doc/trello).