add plaintext request desc

[robbinhan]

I not found about plaintext method example code, so I add this

[thelinuxfoundation]

Thank you for your pull request. Before we can look at your contribution, we need to ensure all contributors are covered by a Contributor License Agreement.

After the following items are addressed, please respond with a new comment here, and the automated system will re-verify.

• One or more commits are not associated with a GitHub user. The email used in the Git commit must match a verified email address in a GitHub user profile. For more information, please see https://help.github.com/articles/why-are-my-commits-linked-to-the-wrong-user/
• User @robbinhan isn't covered by a CLA. They will need to complete the form at https://identity.linuxfoundation.org/projects/cncf

Regards, CLA GitHub bot

[robbinhan]

I signed it with robbinhan215