Add upcoming deprecation to project README

[ryanjwessel]

Describe the feature you'd like:

According to this GitHub Issue, this project is being deprecated in favor of callstack/react-native-testing-library. I think the README should be updated to reflect this so that people that are introducing Testing Library to their React Native project do not use the wrong dependency. Especially since this one is part of the official Testing Library group, it could lead to confusion.

Suggested implementation:

We could add something like this to the top of the README:

---

DEPRECATION WARNING:

This project is in the process of being deprecated in favor of callstack/react-native-testing-library. If you are adding Testing Library to a React Native project, please use that one instead. If you are using this project currently, we are working on a migration guide to assist you in switching dependencies.

---

Describe alternatives you've considered:

N/A

Teachability, Documentation, Adoption, Migration Strategy:

N/A

---

I am happy to put up a Pull Request for this once we determine the exact wording we'd like to use.

[thymikee]

Happy to help with this one. I think the wording should focus on new users for now, to let them know that there's a different library they want to use. Once we have a migration guide, we can link that as well and provide to existing users.

[sibelius]

what are the motivations behind this?

[ryanjwessel]

That makes sense, thanks @thymikee. So, maybe something like this?

Deprecation Warning

This project is being deprecated in favor of callstack/react-native-testing-library. You should use that library instead of this one. A migration guide is in development now for existing users of this library.

[ryanjwessel]

what are the motivations behind this?

@sibelius I learned about this from one of Kent's tweets, and there is some context there.

[thymikee]

Done in c864cd573cd3ac9f40da8540d6ffbc6b8dc6aa52 :)