What is this error "install nuget package Microsoft.Bcl.Build" about?

I see this in all of my projects that reference my core PCL. What is this about? I read the linked document but it's still not clear to me. Everything seems to work without this package.

Warning 1   All projects referencing SDSClient_Core.csproj must install nuget package Microsoft.Bcl.Build. For more information, see http://go.microsoft.com/fwlink/?LinkID=317569. SDSClient_iOS