Add optional support for L-trigger on controllers.

enhancements/L-trigger-mapping/README.md

@@ -0,0 +1,12 @@
+# L-trigger mapping
+
+Some parts of the code might require the pressing of the L-trigger for testing reasons. 
+
+If you need that, alter `controller_sdl.c`.
+In the following line:
+```
+    if (SDL_GameControllerGetButton(sdl_cntrl, SDL_CONTROLLER_BUTTON_LEFTSHOULDER))    pad->button |= Z_TRIG;
+```
+Replace `Z_TRIG` with `L_TRIG`, save and rebuild.
+
+On a DS4, this now means that Z-trigger will be mapped to L2 and the L-trigger to L1.