doc: Add v2 payload formatter note

doc/content/integrations/payload-formatters/javascript.md

@@ -9,6 +9,8 @@ Javascript payload formatters allow you to write your own functions to encode or
 
 > **Note:** Payload formatters use ECMAScript 5 (2009), which has some distinct differences compared to newer, commonly used ECMAScript revisions. See [here](https://www.javatpoint.com/es5-vs-es6) for a quick comparison. Notably, `let`, `const`, and arrow functions are not supported by ES5.
 
+> **Note:** Javascript payload formatters from {{% ttnv2 %}} will work in {{% tts %}}. {{% tts %}} has a new payload formatter signature, but it is also backwards compatible.
+
 Tips:
 - The payload formatters should be simple and lightweight. 
 - Use arithmetic operations and bit shifts to convert binary data to fields.