docs: update README

Author: ohmont

Diff for: src/slack/README.md

@@ -5,7 +5,7 @@ MCP Server for the Slack API, enabling Claude to interact with Slack workspaces.
 ## Tools
 
 1. `slack_list_channels`
-   - List public channels in the workspace
+   - List public or pre-defined channels in the workspace
    - Optional inputs:
      - `limit` (number, default: 100, max: 200): Maximum number of channels to return
      - `cursor` (string): Pagination cursor for next page
@@ -102,7 +102,8 @@ Add the following to your `claude_desktop_config.json`:
       ],
       "env": {
         "SLACK_BOT_TOKEN": "xoxb-your-bot-token",
-        "SLACK_TEAM_ID": "T01234567"
+        "SLACK_TEAM_ID": "T01234567",
+        "SLACK_CHANNEL_IDS": "C01234567, C76543210"
       }
     }
   }
@@ -124,17 +125,26 @@ Add the following to your `claude_desktop_config.json`:
         "SLACK_BOT_TOKEN",
         "-e",
         "SLACK_TEAM_ID",
+        "-e",
+        "SLACK_CHANNEL_IDS",
         "mcp/slack"
       ],
       "env": {
         "SLACK_BOT_TOKEN": "xoxb-your-bot-token",
-        "SLACK_TEAM_ID": "T01234567"
+        "SLACK_TEAM_ID": "T01234567",
+        "SLACK_CHANNEL_IDS": "C01234567, C76543210"
       }
     }
   }
 }
 ```
 
+### Environment Variables
+
+1. `SLACK_BOT_TOKEN`: Required. The Bot User OAuth Token starting with `xoxb-`.
+2. `SLACK_TEAM_ID`: Required. Your Slack workspace ID starting with `T`.
+3. `SLACK_CHANNEL_IDS`: Optional. Comma-separated list of channel IDs to limit channel access (e.g., "C01234567, C76543210"). If not set, all public channels will be listed.
+
 ### Troubleshooting
 
 If you encounter permission errors, verify that: