doc: added comments to README.md file about new camera example UI

[czp13]

Description

From the issue description:

Just some quick info about the new view and example usage (clarify that it should be tested with a mobile device or can be tested with webcam too?)

Added these points to the README.md requested by Miguel, hope it is good like this!

Thanks for the previous comments, and for testing/supporting Miguel on this new UI! 🙇

Fixes # (issue)

• https://github.com/vaadin/form-filler-addon/issues/146

Type of change

• [ ] Bugfix
• [x] Feature

Checklist

• [x] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• [x] I have added a description following the guideline.
• [x] The issue is created in the corresponding repository and I have referenced it.
• [x] I have added tests to ensure my change is effective and works as intended.
• [x] New and existing tests are passing locally with my change.
• [x] I have performed self-review and corrected misspellings.

Additional for Feature type of change

• [x] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.