Docs - Debian package way out dated

[Darkentik]

Hey,
the docs seems to be out dated, because i got errors about deprecated stuff using your install guide for deb package.

`sysmonmaster: ~/ $ apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 10779AB4
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
Executing: /tmp/apt-key-gpghome.3HMnuTDZue/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys 10779AB4
^C
gpg: signal Interrupt caught ... exiting

sysmonmaster: ~/ $ add-apt-repository "deb https://raw.githubusercontent.com/nisabek/icinga2-slack-notifications/master/reprepro general main"
-bash: add-apt-repository: command not found
`

Please update the docs for deb package way.
Thanks