Marking as unmaintained

Currently, the Amethyst Physics team should be focused on Specs-Physics instead of Amethyst-Rhusics since Rhusics itself is unmaintained. If in the future someone steps up to continue work on Rhusics or this repository this situation could change, but for now I believe we should mark this repository as unmaintained. I'm opening this issue to see if any are interested in picking this repo up or are in opposition before I mark the repository as such in the README and description.