AppVeyor Build for dotnet to get pre-release NuGet feed

[SabotageAndi]

As with @aslakhellesoy shortly discussed on gitter, this is an AppVeyor yml that enables pre-release NuGets via an AppVeyor build.
AppVeyor is hosting the feed, so no need of NuGet.org account credentials.

You have to add the project in AppVeyor as described here: https://www.appveyor.com/docs
As the yml- file already exists, it is only the click on the add button.

In the Settings\NuGet section you will find the feed Url. Please update the one in dotnet/README.md to your value.
Also update the urls of the badge.

I assumed that the next version will be 3.3.0. If this is not correct, please update the GherkinVersion in environment.

[aslakhellesoy]

The next version is going to be 4.0.0, but we didn't know that until recently - we thought it was going to be 3.2.1 until we introduced a backwards-incompatible change.

We won't always know what the next version will be, but I guess it's best to assume it will be the last release +1 patch - then update minor or major if we know it will change.

Anyway - please change this to 4.0.0

[SabotageAndi]

Version is updated to 4.0.0. Or should it be another one as the release already happened?

[aslakhellesoy]

The thing is - I don't know what the next release number will be. We have the same problem with java/pom.xml I suppose. Let's assume 4.0.1 for now and we'll bump it if we know we'll bump the minor or major number.

[SabotageAndi]

@aslakhellesoy It is updated to 4.0.1.