Add License to NuGet package

Author: mthesing88

nuget.package/NativeBinaries.nuspec

@@ -7,6 +7,7 @@
     <license type="file">libgit2\libgit2.license.txt</license>
     <projectUrl>https://github.com/libgit2/libgit2sharp.nativebinaries</projectUrl>
     <icon>libgit2\libgit2.png</icon>
+    <license type="expression">MIT</license>
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <description>Native binaries for LibGit2Sharp</description>
     <repository type="git" url="https://github.com/libgit2/libgit2sharp.nativebinaries.git" />