Add a deprecation message to the README directing people to Waffle

[tyler-eon]

Replaces the build status (which theoretically won't be necessary anymore) with a deprecation message. The message includes instructions on migrating to the new project.

[samhamilton]

@kolorahl just saw this pull, as we at Martide are the maintainers of the GCS plugin for Arc we wanted to know if there is any more details as to why this is happening?

[tyler-eon]

@samhamilton Based on my conversations with the maintainer(s) of the Waffle project, it sounded like they took over the code starting at v0.11. To my knowledge, there haven't been any commits to the main branch of this project since about a year ago but the Waffle project is attempting to migrate over issues and PRs and get the code updated. As to why this is the case, I don't have that information.