add basic instructions

README.md

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+## Doom for 3DS
+
+Place the prboom folder on your SD card in the 3DS folder as specified by [3ds Hbmenu](https://github.com/smealum/3ds_hb_menu).
+
+Start button will bring up game menu and exit from menu. Controls in the menu are DPAD and Select/A to select menu item. B button goes back a menu level.
+
+
+##### In game controls
+
+DPAD and circle pad currently act as direction keys. B to select switches/doors etc. A to fire current weapon, Y to cycle weapon. Select toggles automap, Start toggles menu. L & R strafe.
+
+##### Custom IWADs and PWADs
+
+You can use xml shortcuts to supply arguments to prboom - standard command line works.