Windows binary missing from AppVeyor

[jxu]

https://ci.appveyor.com/project/mbojan/isnar/build/artifacts

Artifacts older than 6 months are automatically deleted. For more details see Artifacts retention policy.

if possible upload binary as github release https://help.github.com/en/articles/creating-releases

[mbojan]

The preferred method is to use remotes::install_github() directly. I'll remove the AppVeyor link from the README.