Created brigthness (markdown)

brigthness.md

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+[<- widgets](https://github.com/copycat-killer/lain/wiki/Widgets)
+
+Shows the current level of screen brightness.
+
+	mybrightness = lain.widgets.contrib.brightness()
+
+### input table
+
+Variable | Meaning | Type | Default
+--- | --- | --- | ---
+`backlight` | Card managing backlight | `acpi_video0` 
+`timeout` | Refresh timeout seconds | int | 5
+`settings` | User settings | function | empty function
+
+`settings` can use the string `brightness_now`, which indicates the current brightness level.
+
+### output table
+
+Variable | Meaning | Type
+--- | --- | ---
+`widget` | The textbox | `wibox.widget.textbox`
+`update` | The notification | function
+
+You can control the widget with key bindings like these: [TODO]