Move staging README.md to the right location

[tengqm]

The content in the README.md file is mostly concerned with the client-go
directory. It is better to move it there to avoid confusion.

Special notes for your reviewer:
We may need a new README.md file documenting the purpose and usage of the staging components.

[k8s-ci-robot]

Hi @tengqm. 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 @k8s-bot 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.

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. I understand the commands that are listed here.

[k8s-ci-robot]

Thanks for your pull request. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please follow instructions at https://github.com/kubernetes/kubernetes/wiki/CLA-FAQ to sign the CLA.

Once you've signed, please reply here (e.g. "I signed it!") and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please sign in with your organization's credentials at https://identity.linuxfoundation.org/projects/cncf to be authorized.

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. I understand the commands that are listed here.

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

The following people have approved this PR: tengqm

Needs approval from an approver in each of these OWNERS Files:

• staging/src/k8s.io/client-go/OWNERS

We suggest the following people:
cc @smarterclayton
You can indicate your approval by writing /approve in a comment
You can cancel your approval by writing /approve cancel in a comment

[k8s-reviewable]

This change is 

[tengqm]

I have signed the CLA.

[grodrigues3]

@k8s-bot ok to test

[k8s-ci-robot]

@tengqm: The following test(s) failed:

Test name	Commit	Details	Rerun command
Jenkins verification	3bfebee	link	@k8s-bot verify test this

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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. I understand the commands that are listed here.

[k8s-github-robot]

@tengqm PR needs rebase

[marun]

This PR hasn't been active months, moving to v1.8.

[k8s-github-robot]

This PR hasn't been active in 90 days. Closing this PR. Please reopen if you would like to work towards merging this change, if/when the PR is ready for the next round of review.

cc @pmorie @tengqm

You can add 'keep-open' label to prevent this from happening again, or add a comment to keep it open another 90 days