Add documentation on how to install the Spin shim and release versions

[kate-goldenring]

I've also PRd this documentation to the Shim repo: https://github.com/spinkube/containerd-shim-spin/pull/116. I think duplicating the documentation is fine. We may wish to have the release chart live in only location, though

[tpmccallum]

Sorry about the delay getting to this @kate-goldenring