Add documentation and examples

[j-fuentes]

This adds a decent readme and some other documentation (./docs). It adds a tutorial on how to deploy preflight as a cronjob.

Additionally, it adds a Dockerfile.

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[munnerz]

Instead of using multi stage builds, which don't play well with go's module caching, consider using 'go_container.sh' from 'kind' which makes use of a docker volume to cache go dependencies, giving you host-like performance 😀 this works okay, but is just a bit slow for iterative development: https://github.com/kubernetes-sigs/kind/blob/master/hack/go_container.sh

[j-fuentes]

Will do in a separate PR: #8

[munnerz]

We should get some images published at some point soon to to avoid this 😀

[j-fuentes]

Yes, we need to build images and binaries from Prow. Not done yet.

[munnerz]

/lgtm