Created Android tips and tricks (markdown)

Android-tips-and-tricks.md

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+## Customize keys
+
+You can override key mappings by creating koreader/settings/event_map.lua with the things to override.
+
+#### example 1: override the camera key with a page forward event:
+
+```lua
+return {
+   [27] = "LPgFwd",
+}
+```
+
+#### example 2: use key 200 as the back key
+
+```lua
+return {
+   [200] = "Back",
+}
+```
+
+For more information see https://github.com/koreader/koreader/issues/5558#issuecomment-548993898