Update doc/online_accounts_to_join.md

Co-authored-by: Marco Barilari <38101692+marcobarilari@users.noreply.github.com>

Author: fcerpe

doc/online_accounts_to_join.md

@@ -21,6 +21,8 @@ If that is not the case and you hear lab members talk about events you are not a
 - contact Cathy Friand
 
 !!! TIP
+**Setup Gmail labels**
+
 If you want to have emails from the CPP_LLN mailing list clearly marked in your PRIMARY tab on GMail (and potentially other email platforms too), follow these simple steps: 
 
 - create a label (e.g. CPP_LLN or CPP_LLN_group)