Created Gamepad support (markdown)

Gamepad-support.md

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+Koreader supports gamepad for controlling actions. Commands are:
+
+![](https://user-images.githubusercontent.com/202757/38541203-a50d455c-3c9e-11e8-9cb0-de21cda0ca0e.png)
+
+| | Feature |  Status  |
+| ------ | ------ | ------ |
+| 1 | Left stick & d-pad | Arrow keys (menu navigation) |
+| 2 | Right stick | Page up & down |
+| 3 | A button | Press/enter (also opens bottom reader menu) |
+| 4 | B button | Escape/back |
+| 5 | Left bumper | Page up |
+| 6 | Right bumper | Page down |
+|  | Start/Menu button (not shown) | Open menu |
+
+More info can be found [here.](https://github.com/koreader/koreader/pull/3819)