.packit.yaml: Add packit notification specification

[mulkieran]

/packit build

[packit-as-a-service[bot]]

Congratulations! One of the builds has completed. :champagne:

You can install the built RPMs by following these steps:

• sudo yum install -y dnf-plugins-core on RHEL 8
• sudo dnf install -y dnf-plugins-core on Fedora
• dnf copr enable packit/stratis-storage-stratis-cli-1095
• And now you can install the packages.

Please note that the RPMs should be used only in a testing environment.