Fix no /etc/lsb-release under Debian issue.

Closes #30.

Author: brocaar

docs/getting-started.md

@@ -48,8 +48,10 @@ packages. In order to activate this repository, execute the following
 commands:
 
 ```bash
-source /etc/lsb-release
 sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1CE2AFD36DBCCA00
+
+export DISTRIB_ID=`lsb_release -si`
+export DISTRIB_CODENAME=`lsb_release -sc`
 sudo echo "deb https://repos.loraserver.io/${DISTRIB_ID,,} ${DISTRIB_CODENAME} testing" | sudo tee /etc/apt/sources.list.d/loraserver.list
 sudo apt-get update
 ```