Adds Nuget release badge

This PR adds the nuget version badge. For those who aren't familiar:
> [Nuget](https://www.nuget.org/) is the package manager for the Microsoft development platform including .NET.

I've followed the conventions used by the other badges/shields and used [npm](https://github.com/badges/shields/blob/master/server.js#L374-L403)'s as a base for this one. 

If there is anything you need changed or added let me know. Lastly, if I could be being presumptuous... I've taken the liberty of adding my gravatar to the contributors too.