Document /etc/default/telegraf file

docs/CONFIGURATION.md

@@ -24,6 +24,9 @@ Environment variables can be used anywhere in the config file, simply prepend
 them with $. For strings the variable must be within quotes (ie, "$STR_VAR"),
 for numbers and booleans they should be plain (ie, $INT_VAR, $BOOL_VAR)
 
+When using the `.deb` or `.rpm` packages, you can define environment variables
+in the `/etc/default/telegraf` file.
+
 ## Configuration file locations
 
 The location of the configuration file can be set via the `--config` command