cdk-constructs Improve documentation for Slack integration setup

The README describes adding the channels::history scope to the app.

However, different scopes seem to be required. CloudWatch logs for the Slack notifier Lambda contain the following error message:

{
    "errorType": "Runtime.UnhandledPromiseRejection",
    "errorMessage": "Error: An API error occurred: missing_scope",
    "reason": {
        "errorType": "Error",
        "errorMessage": "An API error occurred: missing_scope",
        "code": "slack_webapi_platform_error",
        "data": {
            "ok": false,
            "error": "missing_scope",
            "needed": "channels:read,groups:read,mpim:read,im:read",
            "provided": "channels:history",
            "response_metadata": {
                "scopes": [
                    "channels:history"
                ],
                "acceptedScopes": [
                    "channels:read",
                    "groups:read",
                    "mpim:read",
                    "im:read"
                ]
            }
        },
        "stack": [
            "Error: An API error occurred: missing_scope",
            "    at Object.platformErrorFromResult (/var/task/node_modules/@slack/web-api/dist/errors.js:50:33)",
            "    at WebClient.apiCall (/var/task/node_modules/@slack/web-api/dist/WebClient.js:491:28)",
            "    at process._tickCallback (internal/process/next_tick.js:68:7)"
        ]
    },
    "promise": {},
    "stack": [
        "Runtime.UnhandledPromiseRejection: Error: An API error occurred: missing_scope",
        "    at process.on (/var/runtime/index.js:37:15)",
        "    at process.emit (events.js:198:13)",
        "    at process.EventEmitter.emit (domain.js:448:20)",
        "    at emitPromiseRejectionWarnings (internal/process/promises.js:140:18)",
        "    at process._tickCallback (internal/process/next_tick.js:69:34)"
    ]
}

It appears that the scopes channels:read, groups:read, mpim:read, im:read are required (instead of channels:history?).

Slack is deprecating a lot of methods in favor of their new Conversations API (see #33). The required scopes might possibly change as well