[Doc] Add link to custom fields video tutorial

docs/Fields.md

@@ -457,7 +457,9 @@ const BookList = () => (
 
 If you don't find what you need in the list of available Fields, you can write your own Field component.
 
-It must be a regular React component, accepting a `source` attribute and retrieving the `record` from the `RecordContext` with the `useRecordContext` hook. React-admin will set the `record` in this context based on the API response data at render time. The field component only needs to find the `source` in the `record` and display it.
+<iframe src="https://www.youtube-nocookie.com/embed/tTNDAssRJhU" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen style="aspect-ratio: 16 / 9;width:100%;margin-bottom:1em;"></iframe>
+
+A custom field must be a regular React component retrieving the `record` from the `RecordContext` with the `useRecordContext` hook. React-admin will set the `record` in this context based on the API response data at render time. If you pass a `source`, the field component needs to find the corresponding value in the `record` and render it.
 
 Let's see an example for an API returning user records with `firstName` and `lastName` properties.