add readme and screenshot

Author: Ehsan Ghorbannezhad

README.md

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+# selectformat
+
+selectformat is an mpv script that allows you to select the
+youtube-dl format of the video on the fly.
+
+![Screenshot](screenshot.jpg)
+
+## Features
+
+- More useful info about the formats are displayed compared to [mpv-youtube-quality](https://github.com/jgreco/mpv-youtube-quality)
+- The formats are properly sorted based on resolution, FPS, dynamic range, codec, bitrate, sample rate and protocol
+- Formats are fetched asynchronously as soon as a network video starts
+- The initially loaded format is pre-selected in the menu
+
+## TODO
+
+- [ ] Fold formats with the same resolution into a submenu
+- [ ] Proper scrolling in case the menu is longer that the screen height