SDL: README-migration.md: Added note about SDL_HasRDTSC removal.

From 321fc1841700a2f9920539f3caa348368fc57f64 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Wed, 11 Oct 2023 01:20:18 -0400
Subject: [PATCH] README-migration.md: Added note about SDL_HasRDTSC removal.

Fixes #8374.
---
 docs/README-migration.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/docs/README-migration.md b/docs/README-migration.md
index c4dd52b5f6e4..7878b1f4d07a 100644
--- a/docs/README-migration.md
+++ b/docs/README-migration.md
@@ -276,6 +276,8 @@ The intrinsics headers (mmintrin.h, etc.) have been moved to `<SDL3/SDL_intrin.h
 
 SDL_Has3DNow() has been removed; there is no replacement.
 
+SDL_HasRDTSC() has been removed; there is no replacement. Don't use the RDTSC opcode in modern times, use SDL_GetPerformanceCounter and SDL_GetPerformanceFrequency instead.
+
 SDL_SIMDAlloc(), SDL_SIMDRealloc(), and SDL_SIMDFree() have been removed. You can use SDL_aligned_alloc() and SDL_aligned_free() with SDL_SIMDGetAlignment() to get the same functionality.
 
 ## SDL_events.h