Update Cake.DNF.Module README: Add link to GitHub Discussions

We've started using [GitHub Discussions](https://github.com/cake-build/cake/discussions) as the preferred communication channel moving forward (instead of Gitter) because it makes it easier to keep track of discussions in a structured way, especially if multiple discussions are happening at the same time. It also allows to search for previous questions/answers, which can be a helpful resource.

As such, we're recommending addin maintainers to update any links to Gitter to point to the GH discussions.


Template:

```markdown
## Discussion

For questions and to discuss ideas & feature requests, use the [GitHub discussions on the Cake GitHub repository](https://github.com/cake-build/cake/discussions), under the [Extension Q&A](https://github.com/cake-build/cake/discussions/categories/extension-q-a) category.

[](https://github.com/cake-build/cake/discussions)
```