docs: Add READMEs for compton, dunst, and zathura

Since my dotfiles are now sorted by individual programs instead of in
groups, I'm trying a new README format. This format should provide more
useful information and be easier to read as well.

compton/README.md

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+# compton
+
+[compton][compton] is a desktop compositor for X11.
+
+## Use Cases
+
+compton can be used to:
+
+- Reduce screen tear in certain environments
+- Add elementary animations such as fading to your desktop
+- Add shadows to your desktop
+
+You should not use compton if:
+
+- Your window manager or desktop environment already has a compositor
+- You are not using X11
+
+[compton]: https://github.com/yshui/compton

dunst/README.md

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+# dunst
+
+[dunst][dunst] is used to display notifications on your desktop.
+
+## Use Cases
+
+dunst can be used to:
+
+- Display messages sent with `notify-send`
+- Strategically add notifications to your rice screenshots
+
+You should not use dunst if:
+
+- Your desktop environment already includes a notification daemon
+
+[dunst]: https://github.com/dunst-project/dunst

zathura/README.md

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+# zathura
+
+[zathura][zathura] is a document viewer that displays PDFs and other files with no GUI.
+
+## Use Cases
+
+zathura can be used to:
+
+- View .pdf and .djvu files
+- View documents with the same color scheme as the rest of your setup
+- Recolor a PDF file with a dark background and light text
+- Strategically add documents to your rice screenshots
+
+You should not use zathura if:
+
+- You want to see page thumbnails
+- You want to see and click on GUI components
+
+[zathura]: https://github.com/pwmt/zathura