Fix package readme file test by adding content to it

It seems the readme file cannot be empty when packing, so add some content to fix the failing test on release builds (i.e. in CI).

src/NuGetizer.Tests/CreatePackageTests.cs

@@ -579,6 +579,7 @@ public void when_creating_package_with_content_file_then_adds_as_content_file()
         public void when_creating_package_with_readme_then_has_readme_metadata()
         {
             var content = Path.GetTempFileName();
+            File.WriteAllText(content, "# Readme Sample");
             task.Contents = new[]
             {
                 new TaskItem(content, new Metadata