Add AppVeyor Content

This PR adds a template for AppVeyor (`appveyor.yml`) to build packages / run specs on Windows. The README is also updated to reflect two methods / platforms.

**Note:** Successful use of this template is dependent on https://github.com/atom/apm/pull/149 being merged (otherwise, you need to specify a full, versioned path for Atom to apm:

> `apm test --path C:\ProgramData\chocolatey\lib\Atom.0.115.0\tools\Atom\atom.exe`
So does appveyor come with chocolatey already installed?