docs: Add READMEs for mpd, mpv, and sxhkd

Contrary to the similarity in naming, mpd and mpv are two completely
different programs that both serve completely different purposes.

mpd/README.md

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+# mpd
+
+[mpd][mpd] is a music player daemon.
+
+## Use Cases
+
+mpd can be used to:
+
+- Manage music independently of which client is being used
+
+You should not use mpd if:
+
+- The music player you use does not rely on mpd
+
+[mpd]: https://github.com/MusicPlayerDaemon/MPD

mpv/README.md

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+# mpv
+
+[mpv][mpv] is a media player without a GUI.
+
+## Use Cases
+
+mpv can be used to:
+
+- Play every video format you encounter
+- Work behind the scenes for larger GUI programs
+- Watch videos with other tiled windows open and without GUI buttons taking up space
+- Strategically position GUI-less videos in your rice screenshots
+
+You should not use mpv if:
+
+- You use a desktop environment with a media player
+
+[mpv]: https://github.com/mpv-player/mpv

sxhkd/README.md

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+# sxhkd
+
+[sxhkd][sxhkd] is a hotkey daemon for X11.
+
+## Use Cases
+
+sxhkd can be used to:
+
+- Bind any key combination to whatever shell command you want
+- Control windows in [bspwm](/bspwm) through `bspc`
+
+You should not use sxhkd if:
+
+- You use a desktop environment that sets keybinds for you
+
+[sxhkd]: https://github.com/baskerville/sxhkd