update readme

README.md

@@ -39,7 +39,9 @@ DDB.config.pretty_json_files = True # Default value
 
 You can specify your own json encoder and decoder if you need to.
 The standard library json module is sufficient most of the time.
-However, alternatives like orjson might be more performant for your use case
+However, alternatives like orjson might be more performant for your use case.
+The encoder function should take a dict and return a str or bytes.
+The decoder function should take a string and return a dict.
 ```python
 DDB.config.custom_json_encoder = None # Default value
 DDB.config.custom_json_decoder = None # Default value