Templating Roslyn Source Code Generators

I want that,when I generate code with Roslyn,to have a template that I can easy modify to generate code . Also,I want to let the user ( the programmer,in this case) modify this template  – maybe he has better ideas than me.

For reading from the RSCG,is pretty simple: Make as an embedded resource and read with

internal class EmbedReader
{
     static Assembly assembly;
     static EmbedReader()
     {
         assembly = Assembly.GetExecutingAssembly();

    }
     public static string ContentFile(string name)
     {
         var resourceName = name;

        using (Stream stream = assembly.GetManifestResourceStream(resourceName))
         using (StreamReader reader = new StreamReader(stream))
         {
             string result = reader.ReadToEnd();
             //this is the content
         }
     }
}

For let the programmer user put his template,put as as additional file in the csproj

<ItemGroup>
         <AdditionalFiles Include=”context.txt”  />

</ItemGroup>

and read with

var file = context.AdditionalFiles.Where(it => it.Path.EndsWith($”{val}.txt”))
                     .Select(it => it.GetText())
                     .FirstOrDefault();

            if (file != null)
             {
                 content = string.Join(Environment.NewLine,file.Lines.Select(it => it.ToString()));
             }

To do both,if the result of the first is empty,read the second (or opposite ? )