feat: add deprecation notice to the README file

This project has been deprecated and is no longer actively maintained.

Please consider using the more generic and actively developed asdf-gl-infra[1] plugin to replace installing and managing the jsonnet-tool using asdf.

1. https://gitlab.com/gitlab-com/gl-infra/asdf-gl-infra

Signed-off-by: Krzysztof Wilczyński kwilczynski@gitlab.com