Update en-GB.ini

installation/language/en-GB/en-GB.ini

@@ -68,6 +68,7 @@ INSTL_DATABASE_PREFIX_MSG="The table prefix must start with a letter, optionally
 INSTL_DATABASE_TYPE_DESC="Select the database type."
 INSTL_DATABASE_USER_DESC="Either a username you created or a username provided by your host."
 INSTL_DATABASE_VALIDATION_ERROR="Check your database credentials, database type, database name or hostname. If you have MySQL 8 installed then please read this <a href=\"https://docs.joomla.org/Joomla_and_MySQL_8#Workaround_to_get_Joomla_working_with_MySQL_8\" target=\"_blank\">wiki</a> for more information."
+INSTL_USERNAME_INVALID="Joomla Super User account name invalid, avoid using special characters like '<, >, %, (, ), &' and quotes"
 
 INSTL_INSTALL_JOOMLA="Install Joomla"
 INSTL_CONNECT_DB="Setup Database Connection"