Setup Slack notifications

[mkondratek]

I think that the things changed a bit since we have setup Slack notification for https://github.com/VirtusLab/git-machete-intellij-plugin

Slack integration's docs are outdated for now. The only working way to achieve the goal seems to be slack orb. See:

• https://github.com/CircleCI-Public/slack-orb/wiki/Setup
• https://circleci.com/blog/circleci-slack-integration/