Add <auto-generated> (#24)

src/ProxyInterfaceSourceGenerator/FileGenerators/PartialInterfacesGenerator.cs

@@ -30,7 +30,7 @@ namespace ProxyInterfaceSourceGenerator.FileGenerators
             var interfaceName = targetClassSymbol.ResolveInterfaceNameWithOptionalTypeConstraints(pd.InterfaceName);
 
             var file = new FileData(
-                $"{pd.FileName}.cs",
+                $"{pd.FileName}.g.cs",
                 CreatePartialInterfaceCode(pd.Namespace, targetClassSymbol, interfaceName, pd.ProxyAll)
             );
 
@@ -39,7 +39,17 @@ namespace ProxyInterfaceSourceGenerator.FileGenerators
             return file;
         }
 
-        private string CreatePartialInterfaceCode(string ns, INamedTypeSymbol targetClassSymbol, string interfaceName, bool proxyAll) => $@"using System;
+        private string CreatePartialInterfaceCode(string ns, INamedTypeSymbol targetClassSymbol, string interfaceName, bool proxyAll) => $@"//----------------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by https://github.com/StefH/ProxyInterfaceSourceGenerator.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//----------------------------------------------------------------------------------------
+
+#nullable enable
+using System;
 
 namespace {ns}
 {{
@@ -51,7 +61,8 @@ namespace {ns}
 
 {GenerateEvents(targetClassSymbol)}
     }}
-}}";
+}}
+#nullable disable";
 
         private string GenerateProperties(INamedTypeSymbol targetClassSymbol, bool proxyAll)
         {