Add NuGet package badge (#1980)

Author: mungojam

Diff for: README.md

@@ -1,6 +1,7 @@
 # LibGit2Sharp
 
-[![CI](https://github.com/libgit2/libgit2sharp/actions/workflows/ci.yml/badge.svg)](https://github.com/libgit2/libgit2sharp/actions/workflows/ci.yml)
+[![CI](https://github.com/libgit2/libgit2sharp/actions/workflows/ci.yml/badge.svg)](https://github.com/libgit2/libgit2sharp/actions/workflows/ci.yml) 
+[![NuGet version (LibGit2Sharp)](https://img.shields.io/nuget/v/LibGit2Sharp.svg)](https://www.nuget.org/packages/LibGit2Sharp/)
 
 **LibGit2Sharp brings all the might and speed of [libgit2](http://libgit2.github.com/), a native Git implementation, to the managed world of .NET**