Add crossplane-cli to krew-index

[displague]

In order to make Crossplane CLI more discoverable and easier to install/upgrade, it should be made available in the Krew Index: https://github.com/kubernetes-sigs/krew-index

Requirements of adding Crossplane CLI to krew include:

• adding the LICENSE file to the release tarball
• creating a PR on krew-index abiding by the publishing guidelines: https://github.com/kubernetes-sigs/krew/blob/master/docs/DEVELOPER_GUIDE.md#publishing-plugins
• adding a kubectl-crossplane-krew wrapper to serve as a single point of entry for a single crossplane plugin (to prevent the need to make a new plugin entry for each subcommand)

Paraphrasing @ahmetb:

Krew doesn't support shipping all of the separate plugins at once. So the recommendation is to:

• create a single entrypoint like kubectl-crossplane-krew
• have this script discover its installation directory (eg $(dirname $(readlink "$0")))
• based on args, it would call $dir/kubectl-crossplane-XXX

The entrypoint bin would be kubectl-crossplane-krew, and so long as the plugin is named crossplane, it will be linked as kubectl-crossplane. Refer to https://github.com/kubernetes-sigs/krew/blob/master/docs/DEVELOPER_GUIDE.md#installing-plugins-locally for local development.

[jbw976]

Has the work also been done to actually add crossplane to the krew index? If not, this issue should be reopened to track that remaining work.