Add installation instructions for Debian

Author: pgrange

README.adoc

@@ -46,7 +46,14 @@ ifndef::backend-manpage[]
 
 === installing on Debian/Ubuntu
 
-*bash_unit* deb package is available in the https://github.com/pgrange/bash-unit_deb/releases[bash-unit_deb repo].
+*bash_unit* deb package is available in the https://github.com/pgrange/bash-unit_deb/releases[bash-unit_deb repo]. You may install it with the following commands:
+
+```bash
+curl https://pgrange.github.io/bash-unit_deb/keys.asc | sudo apt-key add -
+echo deb https://pgrange.github.io/bash-unit_deb/debian/ unstable/ | sudo tee -a /etc/apt/sources.list.d/bash-unit.list
+sudo apt update
+sudo apt install bash-unit
+```
 
 === installing on Archlinux