Created Keymapping (markdown)

Keymapping.md

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+Koreader supports keymapping (i.e. defining different commands to physical buttons on devices with them).
+
+To do this, you need to create document (if it doesn't already exist) `koreader/settings/event_map.lua` and there you can edit key bindings.
+
+One example is shown [here,](https://github.com/koreader/koreader/pull/3862#issue-181100453) with example bellow.
+
+`return {
+   [42] = "Back"
+}`
+
+Number 42 is key value, you need to know values for physical keys. Example for Kobo Forma can be found [here.](https://github.com/koreader/koreader/issues/4446#issuecomment-451639442) Back is command you want that button to execute.
+
+Default keymapping is in file [koreader/frontend/device/kobo/device.lua](https://github.com/koreader/koreader/blob/ea424e621ea88ef0519e86112b0373adb5b6efa2/frontend/device/kobo/device.lua#L240-L250)
+
+```    -- NOTE: For the Forma, with the buttons on the right, 193 is Top, 194 Bottom.
+    self.input = require("device/input"):new{
+        device = self,
+        event_map = {
+            [59] = "SleepCover",
+            [90] = "LightButton",
+            [102] = "Home",
+            [116] = "Power",
+            [193] = "RPgBack",
+            [194] = "RPgFwd",
+        },
+```
+
+For inverting button function you change button commands like this:
+
+```
+return {
+            [194] = "RPgBack",
+            [193] = "RPgFwd",
+ }
+```
+
+More info on this can be found [here.](https://github.com/koreader/koreader/issues/4446)