remove unsecure example of handling creating secrets with sensitive data

Author: sosa-vaadin

articles/control-center/getting-started/upgrade.adoc

@@ -62,13 +62,16 @@ kubectl -n control-center edit configmap control-center
 
 === Secret for Control Center User Credentials
 
-You'll need to set up a secret which holds the email and password of the main Control Center user. These values must be the same as what was entered during the installation process of version 1.0, in the installation wizard. Assuming the email was `[email protected]` and the password was `examplepassword123`, you would execute the following:
+You'll need to set up a secret which holds the email and password of the main Control Center user.
+These values must be the same as what was entered during the installation process of version 1.0, in the installation wizard.
+
+WARNING: To prevent the sensitive values of the secrets being *stored in the terminal command history*, write down the values in files (*without* an extra newline character at the end of the text) and pass those files to `kubectl` to save their content as the secret's values.
 
 [source,bash]
 ----
 kubectl -n control-center create secret generic control-center-user \
 [email protected] \
---from-literal=password=examplepassword123
+--from-file=password=/path/to/password.txt
 ----
 
 You'll have to adjust these values, though, for your email address and password.