rest: add note on content type

* Highlighting the introduction of a new content type to make it clearer and reduce any potential confusion for users.

* zenodo/zenodo-rdm#1078 (comment)
* zenodo/developers.zenodo.org#62 (comment)

docs/reference/rest_api_index.md

@@ -28,6 +28,15 @@ curl -k https://127.0.0.1:5000/api/records?access_token=API-TOKEN
     The `-k` or `--insecure` option here is simply because a certificate is typically not setup locally. In production,
     your `curl` calls shouldn't need this option because you will be using a valid certificate.
 
+_Introduced in InvenioRDM v12_
+When making requests to the new REST API endpoints introduced in v12, you can set the Content-Type header to:
+
+```shell
+Content-Type: application/vnd.inveniordm.v1+json
+```
+
+Including Content-Type ensures that the server correctly interprets your request payloads according to the new API specifications introduced in v12. while the `application/json` Content-Type is still supported, it is recommended to use the new Content-Type to ensure compatibility with future versions of InvenioRDM.
+
 ### Scopes
 
 !!! warning "Work in progress"