docs: reorganize README, add Docker quick start and tested clients

.env.example

@@ -0,0 +1,12 @@
+# Navidrome/Subsonic server URL
+SUBSONIC_URL=http://localhost:4533
+
+# Path where downloaded songs will be stored on the host
+DOWNLOAD_PATH=./downloads
+
+# Deezer ARL token (required)
+# See README.md for instructions on how to get this token
+DEEZER_ARL=your-deezer-arl-token
+
+# Fallback ARL token (optional)
+DEEZER_ARL_FALLBACK=