Document SDL_HINT_RENDER_LINE_METHOD in the release notes

WhatsNew.txt

@@ -8,6 +8,7 @@ This is a list of major changes in SDL's version history.
 General:
 * SDL_RenderGeometryRaw() takes a pointer to SDL_Color, not int. You can cast color data in SDL_PIXELFORMAT_RGBA32 format (SDL_PIXELFORMAT_ABGR8888 on little endian systems) for this parameter.
 * Improved accuracy of horizontal and vertical line drawing when using OpenGL or OpenGLES
+* Added the hint SDL_HINT_RENDER_LINE_METHOD to control the method of line drawing used, to select speed, correctness, and compatibility.
 
 Windows:
 * Fixed size of custom cursors