Added CI/CD using AppVeyor and GitVersion

[mrstebo]

In order to ensure that users of this package know what version of the code they are using, I think that it should have a CI/CD pipeline.

With this setup it will deploy to a pre-release NuGet feed. This allows users to beta test new features. When you then tag a release, then it will publish to NuGet.

These feeds can be set up in AppVeyor.

Instructions for setting up AppVeyor:

1. Create an account on AppVeyor. The easiest option is to sign in using your GitHub account.
2. Authorize AppVeyor so it can see your repositories.
3. Add a New Project and select this repository.
4. Go to badges and replace the badge markdown with the one that you are given.
5. Set up your Environment so that it knows where to put the pre release and release packages.

It is an approach I use for my NuGet packages, and it works really well. What do you think?