message: re-added appveyor build badges to readme - turns out doctest wasn't tested with MSVC 2015 in github actions...
diff: master branch
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  + Windows <a href="https://ci.appveyor.com/project/doctest/doctest/branch/master"><img src="https://ci.appveyor.com/api/projects/status/j89qxtahyw1dp4gd/branch/master?svg=true"></a>
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  - <a href="https://coveralls.io/github/onqtam/doctest?branch=master"><img src="https://coveralls.io/repos/github/onqtam/doctest/badge.svg?branch=master"></a>
  + All <a href="https://github.com/doctest/doctest/actions?query=branch%3Amaster"><img src="https://github.com/doctest/doctest/workflows/CI/badge.svg?branch=master"></a>
  + </td>
  + <td>
  + <a href="https://coveralls.io/github/doctest/doctest?branch=master"><img src="https://coveralls.io/repos/github/doctest/doctest/badge.svg?branch=master"></a>
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  dev branch
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  - All <a href="https://github.com/onqtam/doctest/actions?query=branch%3Adev"><img src="https://github.com/onqtam/doctest/workflows/CI/badge.svg?branch=dev"></a>
  + Windows <a href="https://ci.appveyor.com/project/doctest/doctest/branch/dev"><img src="https://ci.appveyor.com/api/projects/status/j89qxtahyw1dp4gd/branch/dev?svg=true"></a>
  + </td>
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  + All <a href="https://github.com/doctest/doctest/actions?query=branch%3Adev"><img src="https://github.com/doctest/doctest/workflows/CI/badge.svg?branch=dev"></a>
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  <td>
  - <a href="https://coveralls.io/github/onqtam/doctest?branch=dev"><img src="https://coveralls.io/repos/github/onqtam/doctest/badge.svg?branch=dev"></a>
  + <a href="https://coveralls.io/github/doctest/doctest?branch=dev"><img src="https://coveralls.io/repos/github/doctest/doctest/badge.svg?branch=dev"></a>
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