Slack integration documentation is missing a permission scope

[jpuris]

Slack requires chat:write as part of "OAuth & Permissions" for the slack integration to be able to post messages.
If the mentioned scope is not defined, one will receive following error from Slack API

Error: The request to the Slack API failed. The server responded with: {'ok': False, 'error': 'missing_scope', 'needed': 'chat:write:bot', 'provided': 'incoming-webhook,files:write'}

Expected results

Slack integration is working as expected

Actual results

Error is presented about missing scope

How to reproduce the bug

1. Add slack integration as per current instructions
2. Invite the app to the target slack channel
3. Schedule an alert to be posted on specific channel
4. Error. The request to the Slack API failed. The server responded with: {'ok': False, 'error': 'missing_scope', 'needed': 'chat:write:bot', 'provided': 'incoming-webhook,files:write'}

Environment

Preset.io
Version: 0.999.0dev

Checklist

Make sure to follow these steps before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

I've reproduced this issue in preset.io only.