add registration_token in default cfg and README

conduit-example.toml

@@ -38,6 +38,12 @@ max_request_size = 20_000_000 # in bytes
 # Enables registration. If set to false, no users can register on this server.
 allow_registration = true
 
+# A static registration token that new users will have to provide when creating
+# an account. YOU NEED TO EDIT THIS.
+# - Insert a password that users will have to enter on registration
+# - Comment out the line to remove the condition
+registration_token = ""
+
 allow_federation = true
 allow_check_for_updates = true