Update changelog to reflect SDL2 status

changelog

@@ -7,6 +7,13 @@ Version 1.13.2+dev:
      relocate the log file to a path with Unicode characters (bug #22897,
      definitely fixed this time).
    * Fix bug #23108: exclude aborted attacks from statistics
+ * Greatly improved SDL2 support. SDL2 is now the default build.
+   * Fix bug #24138: SDL2 crash on resize after starting a game and returning to title screen
+   * Fix bug #23908: SDL2 SDL_BlitSurface Causes Crashes
+   * Fix bug #23821: Text input is broken in GUI1 under SDL2
+   * Fix bug #23820: SDL 2 alpha blending issues
+   * Fix bug #23918: UI Graphics In OS X El Capitan
+   * Fix bug #18112: Color cursor causes slow mouse movement at menus
 
 Version 1.13.2:
  * Add-ons client: