docs: add external playlists feature documentation

.env.example

@@ -34,6 +34,15 @@ QOBUZ_USER_ID=
 QOBUZ_QUALITY=
 
 # ===== GENERAL SETTINGS =====
+# External playlists support (optional, default: true)
+# When enabled, allows searching and downloading playlists from Deezer/Qobuz
+# Starring a playlist triggers automatic download of all tracks and creates an M3U file
+ENABLE_EXTERNAL_PLAYLISTS=true
+
+# Playlists directory name (optional, default: playlists)
+# M3U playlist files will be created in {DOWNLOAD_PATH}/{PLAYLISTS_DIRECTORY}/
+PLAYLISTS_DIRECTORY=playlists
+
 # Explicit content filter (optional, default: All)
 # - All: Show all tracks (no filtering)
 # - ExplicitOnly: Exclude clean/edited versions, keep original explicit content