Enable AppVeyor testing

[AppVeyor](http://appveyor.com) is a Windows-based CI system much like Travis CI, and also free for open-source projects. I have prepared [a wrapper around r-travis](/krlmlr/r-appveyor) so that AppVeyor can be used to test R packages (and also later deploy them in binary form). It's work in progress, but I think the format of the `appveyor.yml` configuration file is more or less stable.

Example: [Test output](https://ci.appveyor.com/project/krlmlr/rappdirs/build/1.0.77) for this branch.

This pull request installs the AppVeyor configuration file, asks to ignore it for building the R package, and adds a badge to `README.md`. The URL for the badge's image seems to contain some project-specific code, so it should be adapted to that shown in https://ci.appveyor.com/project/hadley/rappdirs/settings/badges (after registering the project).