CVV update for README.md

[patrick-hoban-cko]

Draft updates for CVV tokenisation

Issue

Issue ticket number and link.

Proposed changes

Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request. If it fixes a bug or resolves a feature request, be sure to link to that issue.

Test Steps

If there's any functionality change, please list a step by step guide of how to verify the changes, and/or upload a screen recording for any visible changes.

For instance:

1. Go to the main screen
2. Tap on primary button
3. Verify the image is shown on the screen

Checklist

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• [ ] Reviewers assigned
• [ ] I have performed a self-review of my code and manual testing
• [ ] Lint and unit tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if applicable)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you choose the solution you did and what alternatives you considered, etc...

[patrick-hoban-cko]

Two comments:

• I'm not a dev, so the steps and code snippets may need updating
• Note that we'll update the structure of the ReadMe to follow iOS in a subsequent PR (see https://github.com/checkout/frames-ios/pull/482/files for an example)