Add some help text to the merged runner.

src/tests/Common/XUnitWrapperGenerator/XUnitWrapperGenerator.cs

@@ -334,6 +334,13 @@ private static string GenerateFullTestRunner(ImmutableArray<ITestInfo> testInfos
         }
         builder.AppendLine();
 
+        builder.AppendLine(@"if (args is [""--help"" or ""-h"" or ""/?"" or ""-?""])");
+        using (builder.NewBracesScope())
+        {
+            builder.AppendLine("XUnitWrapperLibrary.Help.WriteHelpText();");
+            builder.AppendLine("return 0;");
+        }
+
         builder.AppendLine("Initialize();");
 
         // Open the stream writer for the temp log.

src/tests/Common/XUnitWrapperLibrary/Help.cs

@@ -0,0 +1,45 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+
+namespace XUnitWrapperLibrary;
+
+public static class Help
+{
+    public static void WriteHelpText()
+    {
+        string help = """
+        Usage: XUnitWrapper [filter] [options]
+
+        filter can be one of the following:
+          * Fully qualified method name: Namespace.ClassName.MethodName
+          * Substring of fully qualified method name: ClassName.MethodName or MethodName
+          * Display Name full string: DisplayName=ArrayMarshalling/SafeArray/SafeArrayTest/SafeArrayTest.dll
+          * Display Name substring: DisplayName~SafeArrayTest.dll
+          * NOTE: This is a subset of the dotnet test filter syntax, documented at https://learn.microsoft.com/en-us/dotnet/core/testing/selective-unit-tests?pivots=xunit
+
+        Display Names:
+          * For a given test, the display name can be one of the following:
+            - For assemblies containing a single [Fact] test method, the display name is the assembly name.
+            - For [Theory] tests, the display name is the fully qualified method name with the parameters for the test case.
+            - If the parameters are computed at runtime, the display name will be the fully qualified method name.
+            - For other tests, the display name is the fully qualified method name.
+
+        Options:
+            --help, -h, /?, -?       Display this help text.
+            --exclusion-list=FILE
+                                     Path to a file containing a list of tests to exclude.
+                                     Each line in the file should be in the following format:
+                                        displayName, reason
+                                     (In the dotnet/runtime repository, this file is typically generated as part of the build process as 'src/tests/issues.targets'.)
+                                     If you are running tests outside the repository context, you may need to create your own exclusion list file in the format shown above.
+        """;
+
+        Console.WriteLine(help);
+    }
+}