docs: remove appveyor badge

[dhmlau]

Since we're no longer using appveyor in CI, so removing the badge in README

Checklist

• [x] DCO (Developer Certificate of Origin) signed in all commits
• [ ] npm test passes on your machine
• [ ] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide
• [ ] API Documentation in code was updated
• [ ] Documentation in /docs/site was updated
• [ ] Affected artifact templates in packages/cli were updated
• [ ] Affected example projects in examples/* were updated

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