update README: add note and improve

[ash-paypay]

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you read and signed the automated Contributor's License Agreement?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Sample

https://github.com/paypay/paypayopa-sdk-python/blob/opa/update-readme-202512/README.md

New Feature Submissions:

1. Does your submission pass tests?
2. Have you lint your code locally prior to submission?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}