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.

at_demos/README.md

Lines 19 to 20 in 76f9961

	1. Add the service file to your app: You can simply copy this service file from the `at_hello_world` application. This file contains helper methods that allow you to implement atProtocol functionality with just a couple of lines of code.
	2. Add the configuration file to your app: Again, feel free to copy this from the `at_hello_world` application. This file contains variables that allow you to use the virtual environment. Make sure that the `ROOT_DOMAIN` string is set to `vip.ve.atsign.zone` and you have a unique name for the `NAMESPACE` of your app!

- 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.