Added AppVeyor to the list of CI/CD working with Gitea

[FeodorFitsner]

Hi guys,

Thanks for the creating and supporting Gitea!

We've just announced a new release of AppVeyor with built-in Gitea support: https://www.appveyor.com/blog/2019/09/05/gitea-receives-first-class-support-in-appveyor/

I thought it would be great to let Gitea community know that there is one more CI/CD tool working with Gitea hence this PR updating the docs.

Thank you!

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@85f5654). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #8104   +/-   ##
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  Coverage          ?   41.78%           
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  Files             ?      481           
  Lines             ?    64425           
  Branches          ?        0           
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  Hits              ?    26917           
  Misses            ?    34039           
  Partials          ?     3469

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[sapk]

Make LGTM work.