Slack app must be added into Slack channel before invoking the command

When we invoke slash command without adding the Slack app into Slack channel, the following error happens in
posting the message from Lambda function to Slack:
```json
{ "ok": false, "error": "channel_not_found" }
```

I suggest to add instruction of adding the app to the channel at the beginning of the following section.

https://github.com/aws-samples/amazon-interactive-slack-app-starter-kit?tab=readme-ov-file#testing-instructions

Open the README's Testing Instructions section at the linked documentation anchor. Add the instruction to add the Slack app to the target channel before invoking the slash command, then review the rendered section to confirm the setup order is clear.

Written by the indexing model from the issue text.