Added "auto-generated" tag which is recognized by Visual Studio at the top of each generated C# file.

Auto-generated files are not included in automatic code-cleanup and/or inspection processes.

Author: mhoogveld

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/csharp/CSharpGenerator.java

@@ -614,7 +614,9 @@ private CharSequence generateFileHeader(final String packageName)
         }
 
         return String.format(
-            "/* Generated SBE (Simple Binary Encoding) message codec */\n\n" +
+            "// <auto-generated>\n" +
+            "//     Generated SBE (Simple Binary Encoding) message codec\n" +
+            "// </auto-generated>\n\n" +
             "#pragma warning disable 1591 // disable warning on missing comments\n" +
             "using System;\n" +
             "using Org.SbeTool.Sbe.Dll;\n\n" +