Add SDL3 function name (#719)

* Add SDL3 function name

See https://github.com/libsdl-org/SDL/blob/f40f272107ff9e8cfe96f3a531944e7cc411cee6/test/testcontroller.c#L1945

README.md

@@ -5,15 +5,20 @@ A community sourced database of game controller mappings to be used with SDL2 Ga
 # Usage
 Download gamecontrollerdb.txt, place it in your app's directory and load it.
 
-For example :
-```
+SDL2:
+```c
 SDL_GameControllerAddMappingsFromFile("gamecontrollerdb.txt");
 ```
 
-The database is compatible with SDL v2.0.10 and newer. Older SDL versions are no longer supported.
+SDL3:
+```c
+SDL_AddGamepadMappingsFromFile("gamecontrollerdb.txt");
+```
+
+The database is compatible with SDL v2.0.10 and newer.
 
 ## Create New Mappings
-A mapping looks like this :
+A mapping looks like this:
 ```
 030000004c050000c405000000010000,PS4 Controller,a:b1,b:b2,back:b8,dpdown:h0.4,dpleft:h0.8,dpright:h0.2,dpup:h0.1,guide:b12,leftshoulder:b4,leftstick:b10,lefttrigger:a3,leftx:a0,lefty:a1,rightshoulder:b5,rightstick:b11,righttrigger:a4,rightx:a2,righty:a5,start:b9,x:b0,y:b3,platform:Mac OS X,
 ```