Add basic SpotDL docs

docs/SpotDL.md

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+# Basic Usage
+To download a track, album, or artist from Spotify, use:
+`spotdl <url e.g. https://open.spotify.com/artist/3q7HBObVc0L8jNeTe5Gofh?si=fd6100828a764c3b>`
+
+This is non-interactive and works programmatically.
+
+# Music Library Integration
+To make updating my library easier, each "Artist" folder has a file called `spot.txt` which contains only the Spotify URL for that artist. This makes it possible to run a loop similar to the following:
+
+```sh
+cd /path/to/music/library/artists
+for artist in */; do
+  cd "$(pwd)/$artist" && 
+  spotdl $(cat spot.txt) &&
+  cd ..
+done
+```
+
+# Links
+[coritsky/spotdl on Dockerhub](https://hub.docker.com/r/coritsky/spotdl)
+[Spotdl on GitHub](https://github.com/spotDL/spotify-downloader/)