Adds simple documentation for tide

[sebastienvas]

Moved from #7457 per demand in #5919

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: sebastienvas
To fully approve this pull request, please assign additional approvers.
We suggest the following additional approver: spxtr

Assign the PR to them by writing /assign @spxtr in a comment when ready.

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• prow/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-ci-robot]

Hi @sebastienvas. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[ingvagabund]

@sebastienvas current README for the Tide is here: https://github.com/kubernetes/test-infra/blob/master/prow/tide/README.md. Feel free to extend it or modify it if you find any inconsistencies.