Add logo

Add logo from http://go.microsoft.com/fwlink/?LinkID=288890

Consistent with other existing Microsoft packages. 

For example, https://www.nuget.org/packages/Microsoft.ServiceFabric.FabricTransport.Internal/

Author: PerthCharern

src/Microsoft.OpenApi.Readers/Microsoft.OpenApi.Readers.csproj

@@ -2,7 +2,7 @@
     <PropertyGroup>
         <TargetFrameworks>net46; netstandard2.0</TargetFrameworks>
         <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
-        
+        <PackageIconUrl>http://go.microsoft.com/fwlink/?LinkID=288890</PackageIconUrl>
         <PackageProjectUrl>https://github.com/Microsoft/OpenAPI.NET</PackageProjectUrl>
         <PackageLicenseUrl>https://raw.githubusercontent.com/Microsoft/OpenAPI.NET/master/LICENSE</PackageLicenseUrl>
         <PackageRequireLicenseAcceptance>true</PackageRequireLicenseAcceptance>