Add plugins document

[Mini256]

• [x] size
• [x] assign
• [x] milestone
• [x] needs-rebase
• [x] release-note
• [ ] welcome
• [x] dco
• [x] rerere[en]
• [ ] branchprotector

[Rustin170506]

/help

Need more documentation and translations!

[ti-chi-bot]

@hi-rustin: This request has been marked as needing help from a contributor.

Please ensure the request meets the requirements listed here.

If this request no longer meets these requirements, the label can be removed by commenting with the /remove-help command.

In response to [this](https://github.com/ti-community-infra/tichi/issues/423): >/help > >Need more documentation and translations! Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[ti-chi-bot]

Issues go stale after 90d of inactivity. Mark the issue as fresh with /remove-lifecycle stale. Stale issues rot after an additional 30d of inactivity and eventually close. If this issue is safe to close now please do so with /close. Send feedback to sig-community-infra or Mini256. /lifecycle stale

[Mini256]

/remove-lifecycle stale

[ti-chi-bot]

Issues go stale after 90d of inactivity. Mark the issue as fresh with /remove-lifecycle stale. Stale issues rot after an additional 30d of inactivity and eventually close. If this issue is safe to close now please do so with /close. Send feedback to sig-community-infra or Mini256. /lifecycle stale

[ti-chi-bot]

Stale issues rot after 30d of inactivity. Mark the issue as fresh with /remove-lifecycle rotten. Rotten issues close after an additional 30d of inactivity. If this issue is safe to close now please do so with /close. Send feedback to sig-community-infra or Mini256. /lifecycle rotten