Add information about running user for commands

Currently, if one follows the install tutorial and enable autostart, he can miss the fact that the running user will be duniter and that running commands on his user won't do any good

content/pages/wiki/duniter/commands.md

@@ -5,6 +5,13 @@ Slug: commands
 Authors: cgeek
 Here is a guide to the different commands of the `duniter` command-line executable.
 
+## Foreword
+
+These commands should be executed by the user that will actually __run__ the node. For example, if you use systemd to automatically run your node on startup, it will run under the user `duniter`. In that case, the commands below should be run as follow:
+```
+sudo -u duniter <cmd>
+```
+
 ## Initialize its node
 
 ### `wizard key`