Add Compatibility Notes section to readme.txt

[daledupreez]

Fixes STRIPE-828
Fixes #4827

Changes proposed in this Pull Request:

This PR explores the addition of a new Compatibility Notes section to our readme.txt file as a way to highlight important features and compatibility issues across releases.

Testing instructions

• Inspection - check that the listed features line up with the core features from releases over the last ~6 months.
  • You can confirm by checking the full changelog in changelog.txt, or by reviewing the release notes for each release, e.g. https://github.com/woocommerce/woocommerce-gateway-stripe/blob/release/10.1.0/readme.txt for 10.1.0
• Basic rendering check - you can paste the contents of readme.txt into wpreadme.com and check that it looks OK.

---

• Covered with tests (or have a good reason not to test in description ☝️)
• [N/A] Tested on mobile (or does not apply)

Changelog entry

• This Pull Request does not require a changelog entry. (Comment required below)

Changelog Entry Comment

Comment

This PR only adds content to the readme.txt without making any functional changes to the plugin code.

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)