Remove SDL_CalculateGammaRamp.

docs/README-migration.md

@@ -4,6 +4,10 @@ This guide provides useful information for migrating applications from SDL 2.0 t
 
 We have provided a handy Python script to automate some of this work for you [link to script], and details on the changes are organized by SDL 2.0 header below.
 
+## SDL_pixels.h
+
+SDL_CalculateGammaRamp has been removed, because SDL_SetWindowGammaRamp has been removed as well due to poor support in modern operating systems (see [SDL_video.h](#sdl_videoh)).
+
 ## SDL_rwops.h
 
 SDL_RWFromFP has been removed from the API, due to issues when the SDL library uses a different C runtime from the application.