Update readme text and upload new screenshots

[johndcoy]

Changes proposed in this Pull Request:

Fulfill Stripe's request to update the content on the readme.txt to reflect new content for WordPress.org , along with new screenshots.

Testing instructions

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• [ ] Covered with tests (or have a good reason not to test in description ☝️)
• [ ] Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• [ ] Tested on mobile (or does not apply)

Post merge

• [ ] Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• [ ] Added the needs docs label (or does not apply)
• [ ] Included this PR in the Release Thread scope (or does not apply)