title: Create nuget package with multiple DLLs
body: <p>Let's say I have a project with this structure:</p>
  
  <pre><code>MyLibrary\
    MyLibrary.sln
    MyLibrary.Core\
      MyLibrary.Core.csproj
    MyLibrary.Extensions\
      MyLibrary.Extensions.csproj
    MyLibrary.Tests\
      MyLibrary.Tests.csproj
  </code></pre>
  
  <p>I want to create a single NuGet package which packages MyLibrary.Core.dll and MyLibrary.Extensions.dll.  I can't seem to figure out how to get NuGet to do this.  I've tried building a spec file manually and I've tried building one using "nuget spec MyLibrary.Core.csproj".  I've tried adding all of the DLLs to a lib/ folder which I understand to be the convention-based mechanism for adding DLLs to the package.  In every case I can get the MyLibary.Core.dll to get into the package but the MyLibrary.Extensions.dll does not end up packaged along with it.</p>
  
  <p>TLDR: What is the best practice for creating a NuGet package with multiple projects / assemblies?  Is there a tutorial out there that focuses on this?  The tutorials I've found all focus on simple single-project demos.</p>