slack bridge: Update doc for the new webhook based Slack Bridge.

This commit updates the Slack Bridge doc, primarily
guiding the user to use our Slack Webhook integration.

Author: PieterCK

zulip/integrations/bridge_with_slack/README.md

@@ -5,30 +5,42 @@ This is a bridge between Slack and Zulip.
 ## Usage
 
 ### 1. Zulip endpoint
+
 1. Create a generic Zulip bot, with a full name like `Slack Bot`.
-2. (Important) Subscribe the bot user to the Zulip stream you'd like to bridge your Slack
-   channel into.
-3. In the `zulip` section of the configuration file, enter the bot's `zuliprc`
+
+2. (Important) Subscribe the bot user to the Zulip channel you'd like to bridge
+   your Slack channel into.
+
+3. Create a [Slack Webhook integration bot](https://zulip.com/integrations/doc/slack)
+   to get messages form Slack to Zulip.
+
+4. In the `zulip` section of the configuration file, fill `integration_bot_email`
+   with **Integration bot**'s email. Note that this is the bot you created in
+   step 3 and not in step 1.
+
+5. Also in the `zulip` section, enter the **Generic bot's** `zuliprc`
    details (`email`, `api_key`, and `site`).
-4. In the same section, also enter the Zulip `stream` and `topic`.
+
+6. Moving to over the `channel_mapping` section, enter the Zulip `channel` and `topic`.
+   Make sure that they match the same `channel` and `topic` you configured in steps 2
+   and 3.
 
 ### 2. Slack endpoint
-1. Make sure Websocket isn't blocked in the computer where you run this bridge.
-   Test it at https://www.websocket.org/echo.html.
-2. Go to https://api.slack.com/apps?new_classic_app=1 and create a new classic
-   app (note: must be a classic app). Choose a bot name that will be put into
-   bridge_with_slack_config.py, e.g. "zulip_mirror". In the process of doing
-   this, you need to add oauth token scope. Simply choose `bot`. Slack will say
-   that this is a legacy scope, but we still need to use it anyway. The reason
-   why we need the legacy scope is because otherwise the RTM API wouldn't work.
-   We might remove the RTM API usage in newer version of this bot. Make sure to
-   install the app to the workspace. When successful, you should see a token
-   that starts with "xoxb-...". There is also a token that starts with
-   "xoxp-...", we need the "xoxb-..." one.
-3. Go to "App Home", click the button "Add Legacy Bot User".
-4. (Important) Make sure the bot is subscribed to the channel. You can do this by typing e.g. `/invite @zulip_mirror` in the relevant channel.
-5. In the `slack` section of the Zulip-Slack bridge configuration file, enter the bot name (e.g. "zulip_mirror") and token, and the channel ID (note: must be ID, not name).
+
+1. Go to the [Slack Apps menu](https://api.slack.com/apps) and open the same Slack app that
+   you have use to set up the Slack Webhook integration previously.
+
+2. Navigate to the "OAuth & Permissions" menu and scroll down to the "Scopes"
+   section in the same page and make sure:
+   - "Bot Token Scopes" includes: `chat:write`
+   - "User Tokens Scopes" includes: `chat:write`
+
+3. Next, also in the same menu find and note down the "Bot User OAuth Token".
+   It starts with "xoxb-..." and not "xoxp" (legacy).
+
+4. In the `slack` section of the Zulip-Slack bridge configuration file, enter the bot name
+   (e.g "zulip_mirror"), token (e.g xoxb-...), and the channel ID (note: must be ID, not name).
 
 ### Running the bridge
 
-Run `python3 run-slack-bridge`
+Run Slack Bridge: `python3 run-slack-bridge`