add README.md

Author: Valent-in

README.md

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+# RetroPad Editor
+Online tool to create and edit onscreen gamepads for RetroArch.
+
+https://valent-in.github.io/retropad-editor/
+
+Features:
+- Create, move, resize buttons of virtual gamepad.
+- Multiple layers; auto switch orientation for portrait overlays.
+- Fix overlay aspect ratio.
+- Can use external image resources.
+
+Config file and images must be stored in same folder.
+On most Android devices RetroArch will open config only from internal memory even acces to sdcard is granted.
+- This editor can load but NOT save configs with image paths (img/A.png will be saved as A.png)
+- Does not support old (integer) format.
+---
+Used media resources from https://github.com/libretro/common-overlays (button images) and https://github.com/libretro/RetroArch (icon).