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

Fixes #8374.

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