feat: documented the changes made in PR 1633

[qcdyx]

Summary:

Closes #1742

Expected behavior:

User should be able to see documentation about user agent header in the "Using web-based validator" heading of the README

Please make sure these boxes are checked before submitting your pull request - thanks!

• [x] Run the unit tests with gradle test to make sure you didn't break anything
• [ ] Add or update any needed documentation to the repo
• [x] Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• [x] Linked all relevant issues
• [ ] Include screenshot(s) showing how this pull request works and fixes the issue(s)

[qcdyx]

@qcdyx Can we include the actual user agent string in the docs?

@emmambd It's a good idea! Done!