Update README.md (minor edits for grammar and content)

[unfulvio-godaddy]

Summary

Just a couple of edits for grammar and content which I missed while reviewing #217

• There is no "Distributor"
• Good to reference that we run tests against latest PHP as well
• CHANGELOG > Changelog for consistency with other linked md files
• "A complete listing of all notable changes are documented" > shouldn't it be "A complete listing of all notable changes is documented" since the subject is "a complete listing"?

Details

Contributor checklist

• [x] I agree to follow this project's Code of Conduct.
• [x] I have updated the documentation accordingly
• [x] I have added tests to cover changes introduced by this pull request
• [x] All new and existing tests pass

Testing

Reviewer checklist

- [x] Code changes review - [x] Documentation changes review - [ ] Unit tests pass - [x] Static analysis passes

[unfulvio-godaddy]

thanks for checking @agibson-godaddy mind re-approving?

[coveralls]

coverage: 59.84%. first build when pulling 1638cb2193c534fdd415dfb0d7fdf97837b68298 on chore/update-wp-mock-readme into 4b56a8cac2abd8dfc4287a6235e5ca1974f5c570 on trunk.

[unfulvio-godaddy]

looks like there might be a flaky test, but I believe I'm having this fixed in another PR