Update Cake.Xamarin README: Add link to GitHub Discussions

[augustoproiete]

We've started using GitHub 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:

## 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.

[![Join in the discussion on the Cake repository](https://img.shields.io/badge/GitHub-Discussions-green?logo=github)](https://github.com/cake-build/cake/discussions)