chore: add link to configuration file in README

[drecali]

- What I did Added a link to the configuration and the service files mentioned in the README paragraphs below. As someone new to Flutter, I wasn't sure where these files are usually kept, so I thought it's helpful to link users directly to it.

https://github.com/atsign-foundation/at_demos/blob/76f9961ce3d0805c967d7f77906f0d354a26d358/README.md?plain=1#L19-L20

- How I did it Links to both files were added as relative links with minimal rephrasing of the existing text.

For the service file, I looked for a file that seemed to contain helper methods to allow users to implement atProtocol functionality. If this is not the right file, please let me know.

For the configuration file, I searched for a file with the variables mentioned above: ROOT_DOMAIN and NAMESPACE. It had the relative path: at_hello_world/lib/utils/constants.dart.

- How to verify it Please confirm these are the right files. You can click the links to confirm they work.

If I missed something or need to do more, please let me know.

- Description for the changelog Add a link to the configuration and the service files mentioned in the README.