Update login credentials doc in sample config.

config.toml.sample

@@ -5,11 +5,15 @@
 # port, use port 80 (this will require running with elevated permissions).
 address = "localhost:9000"
 
-# IMPORTANT: This is only used during installation (--install) for creating
-# the superadmin user in the database.
+# ###### IMPORTANT ######
+# Username and password are only used during installation (--install)
+# for creating the superadmin user in the database.
 #
 # After installation, login to the admin dashboard, update the superadmin's
-# user profile, and empty these two variables from the config file.
+# user profile with the necessary details and remove these username and
+# password from here.
+#
+# Username should be min 3 chars and Password should be min 8 chars.
 admin_username = "listmonk"
 admin_password = "listmonk"