docs: guideline 3

docs/guidelines/documentation-guidelines.md

@@ -57,6 +57,13 @@ NB: this does not apply to file names or other text that are not meant to be cop
 
 > Example: this document is named `docs/guidelines/documentation-guidelines.md` instead of `docs/guidelines/documentation.md` because it could cause confusion with `docs/documentation.md`.
 
+3. Commands SHOULD start with a caret: (is that the tehnical term ?)
+
+    - `$` if the command should be run as the current user 
+    - `#` if the command should be run as root
+
+    > Example: To build in docker, `$ docker build .`
+
 More TODO
 
 ## License