Add AppVeyor support

[thomasfaingnaert]

I added AppVeyor configuration to build OpenPose automatically on Windows (CPU version).
A ZIP with the build artifacts is also available (e.g. https://ci.appveyor.com/api/buildjobs/na0fs1ws5sdgke3q/artifacts/openpose.zip)

[gineshidalgo99]

Thanks! I'll check it!

[gineshidalgo99]

I am not familiar with it, so could you share some additional instructions on how to make this work? I.e., after I accept the PR, what else is required for it to work? Thanks!

[thomasfaingnaert]

You need to add AppVeyor as a GitHub app to this repo as well.
I believe if you go to https://www.appveyor.com and log in using your GitHub account, it should automatically walk you through the setup process.
Once AppVeyor is registered, Windows builds should work automatically.
From the build description (e.g. https://ci.appveyor.com/project/thomasfaingnaert/openpose/builds/23122680), you can see the log and the generated build artifacts.
You may also want to add a status badge to the README (see https://www.appveyor.com/docs/status-badges/).
Let me know if I can help with anything else.

[gineshidalgo99]

Added, configured, and build icon in the README, thanks a lot!!!