chore: add wording for period inclusion of unstable fields

README.md

@@ -136,7 +136,7 @@ of the returned object.
 Note that due to end-to-end encryption, FormSG servers are unable to verify the data format.
 
 However, the `decrypt` function exposed by this library [validates](https://github.com/opengovsg/formsg-javascript-sdk/blob/master/src/util/validate.ts) the decrypted content and will **return `null` if the
-decrypted content does not fit the schema displayed below.**
+decrypted content does not contain all of the fields displayed in the schema below.**
 
 | Key         | Type     | Description                                                                                              |
 | ----------- | -------- | -------------------------------------------------------------------------------------------------------- |
@@ -146,6 +146,8 @@ decrypted content does not fit the schema displayed below.**
 | fieldType   | string   | The type of field for the question.                                                                      |
 | \_id        | string   | A unique identifier of the form field. WARNING: Changes when new fields are created/removed in the form. |
 
+Additional internal fields may be included from time to time, which will then be published above once it is stable for public consumption.
+
 The full schema can be viewed in
 [`validate.ts`](https://github.com/opengovsg/formsg-javascript-sdk/tree/master/src/util/validate.ts).