Update debian.md

added local login info

docs/sources/installation/debian.md

@@ -100,6 +100,8 @@ This will start the `grafana-server` process as the `grafana` user,
 which was created during the package installation. The default HTTP port
 is `3000` and default user and group is `admin`.
 
+Default login and password `admin`/ `admin`
+
 To configure the Grafana server to start at boot time:
 
 ```bash