Windows-based CI for is currently done on Simverge's private GitLab server.

• Create an AppVeyor configuration file (appveyor.yml) for building and testing pdal-c in Windows, using the existing GitLab CI (.gitlab-ci.yml) and PDAL's appveyor.yml as examples
• Add an AppVeyor build badge to the top-level README.md

There is already an AppVeyor project for Simverge/pdal-c under @jimmysoda's account. We can use this setup to make sure the AppVeyor configuration works as expected when complete.