Improve and document local dev experience

[sethetter]

• Utility for simulating event callback requests from Slack
• Documentation

For simulating event calls from Slack locally..

• ./simulate-app-mention.ts "@bot some message"
• The util sends a fake app_mention event payload to the local server.
  • Hardcodes the channel to respond in to #bot-testing.
  • Optionally a message thread ID can be provided to the util to test threaded replies.
• The actual response is sent to the #bot-testing slack channel, or the provided thread.

Only pre req for someone to use this locally is a SLACK_TOKEN that allows posting as the @bot (test) app.

[sethetter]

Closed by 68d6e0b