WIP: Placeholder for ExecutionHook doc

[xing-yang]

This is a placeholder for ExecutionHook documentation.
Tracking issue: kubernetes/enhancements#962
KEP: https://github.com/kubernetes/enhancements/blob/master/keps/sig-storage/20190120-execution-hook-design.md

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[sftim]

@xing-yang

Suggested change

	{{< feature-state for_k8s_version="v1.15" state="alpha" >}}

Here's a Hugo shortcode that you can use to mark this as an alpha feature (if you want).

[xing-yang]

Sure. Thanks!

[makoscafee]

/milestone 1.15

[xing-yang]

Since all development work for ExecutionHook is out of tree, I was told this doc PR is not needed in-tree. So I'm going to close this PR.