Add missing step to the CI Slack notifications guide

[mkleszcz]

Documentation is

• Missing
• Outdated
• Confusing
• Not sure?

Explain in detail

Currently in the How to configure CI/CD Slack notifications? guide there is a missing step in the case when channel is private. In this case there is a requirement to invite the @AWS bot into the channel.

It will be also good to advice to change the settings of the bot because by default it will group all the notifications in the single thread if messages comes from the same day.

Your Suggestion for Changes

Add an additional step how to invite the bot to the private slack channel along with the note about the bot configuration.