Update README

README.md

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 # Lumen
 
-Set screen brightness based on contents.
+Lumen is a menu bar application for macOS that magically sets the screen
+brightness based on your screen contents.
 
-Control it by just using your brightness keys!
+**You control Lumen simply by using your brightness keys - it'll automatically
+learn your preferences.**
+
+Lumen will automatically brighten the screen when you're looking at a dark
+window (for example, a full-screen terminal) and it'll automatically darken the
+screen when you're looking at a bright window (for example, a web browser).
+This makes for a much more pleasant experience, especially at night.
+
+## Demo
+
+![Demo][demo]
+
+Without Lumen, web pages are too bright, and the terminal is too dark to read.
+With Lumen, it's perfect.
+
+## Download
+
+This project uses the GitHub Releases feature. **You can find pre-built
+binaries [here][releases].**
+
+As of now, releases are not signed with a developer key. To open unsigned
+applications, use [this method][opening-unsigned].
+
+You can also download and compile the code yourself, if you feel like it.
+
+**Note: Lumen isn't supposed to work with auto brightness enabled, so be sure
+to disable the "automatically adjust brightness as ambient light changes"
+feature in System Preferences.**
+
+## Contributing
+
+Feature requests, bug reports, and pull requests are all appreciated.
 
 ## License
 
 Copyright (c) 2015-2016 Anish Athalye. Released under GPLv3. See
 [LICENSE.txt][license] for details.
 
+[demo]: assets/demo.gif
+[opening-unsigned]: https://support.apple.com/kb/ph14369
+[releases]: https://github.com/anishathalye/lumen/releases
 [license]: LICENSE.txt