slack bridge: Update doc for current version of making Slack bot user.

Author: rht

zulip/integrations/bridge_with_slack/README.md

@@ -15,9 +15,19 @@ This is a bridge between Slack and Zulip.
 ### 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". 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. (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.
-4. In the `slack` section of the configuration file, enter the bot name (e.g. "zulip_mirror") and token, and the channel ID (note: must be ID, not name).
+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).
 
 ### Running the bridge