add README and screenshot

README.md

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+#PyBrite, an xrandr adjustment tool in Python/Tk
+
+![screenshot](https://github.com/mdoege/PyBrite/raw/master/screenshot.png "PyBrite screenshot")
+
+##Usage
+
+The top two buttons adjust brightness, the bottom two adjust gamma (contrast).
+
+The idea is to keep this window in the corner of the screen permanently so you can adjust brightness quickly.
+
+Lowering brightness results in a washed out, low contrast image. Therefore, you should also lower gamma a bit to boost contrast at lower brightness levels.
+
+Note that after waking up from sleep, the system will default to 100% brightness and gamma 1, so you have to click the PyBrite buttons once for the adjustments to take effect.
+
+Also note that other tools that adjust gamma via xrandr, such as redshift, will interfere with PyBrite.
+
+##Files
+
+Your brightness and gamma settings are saved in "~/.config/pybrite".
+