README.md file still references `containerd-shim-spin`

[tpmccallum]

The README.md still has a few references to containerd-shim-spin after spin shim was removed from the repo.

The first is in the Quickstart section:

Perhaps this Quickstart link can stay in the README.md and point to the Quickstart in SpinKube?

The others are in the instructions:

Perhaps the instructions from this SpinKube documentation page can replace the instructions pictured above.

Thanks
Tim