Add a TODO.md file and list the implementation status

TODO.md

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+# TODO
+- [ ] Add missing model properties
+- [ ] Make a `model` class and add the following methods to it:
+  - [ ] `_check_api_access()`
+  - [ ] `_resource_not_found()`
+- [ ] Test `XML` support.
+- [ ] Should `CoverArt` be removed?
+- [ ] Determine if video and non-ID3 endpoints will be supported.
+  -  [ ] If not the Video and NonID3 checks in the `Song` model should be removed.
+- [ ] Implement missing endpoints.
+- [ ] Improve error handling:
+- [ ] Add the `subsonic.system.check_subsonic_extension()` method.
+- [ ] Check and rewrite all docstrings taking care about raising exceptions.
+- [ ] Spin up a `MkDocs` documentation.
+
+## Implementation status
+The final objetive of Knuckles to be a fully compatible implementation wrapper around the [OpenSubsonic API Spec](https://opensubsonic.netlify.app/), a superset of the [Subsonic API Spec](https://subsonic.org/pages/api.jsp) that tries to improve and extend the API without breaking changes.
+
+### Requests verbs
+- [x] `GET` verb
+- [x] `POST` verb
+
+### Response format
+- [x] `JSON`
+- [ ] `XML`
+- [ ] `JSONP`
+
+### Authentication
+- [ ] Clear text _(Should be tested globally like the verbs and not in test by itself)_
+- [ ] Hex encoded
+- [x] Authentication token
+
+### Error handling
+- [x] Error codes
+
+### Endpoints
+#### System
+- [x] `ping`
+- [x] `getLicense`
+- [x] `getOpenSubsonicExtensions`
+
+#### Browsing
+- [x] `getMusicFolders` REMOVE?
+- [ ] `getIndexes` **[MISSING]**
+- [ ] `getMusicDirectory` **[MISSING]**
+- [x] `getGenres`
+- [x] `getArtists`
+- [x] `getArtist`
+- [x] `getAlbum`
+- [x] `getSong`
+- [ ] `getVideos` **[MISSING]**
+- [ ] `getVideoInfo` **[MISSING]**
+- [ ] `getArtistInfo` **[MISSING]**
+- [x] `getArtistInfo2`
+- [ ] `getAlbumInfo` **[MISSING]**
+- [x] `getAlbumInfo2`
+- [ ] `getSimilarSongs` **[MISSING]**
+- [ ] `getSimilarSongs2` **[MISSING]**
+- [ ] `getTopSongs` **[MISSING]**
+
+#### Album/song lists
+- [ ] `getAlbumList` **[MISSING]**
+- [ ] `getAlbumList2` **[MISSING]**
+- [ ] `getRandomSongs` **[MISSING]**
+- [ ] `getSongsByGenre` **[MISSING]**
+- [ ] `getNowPlaying` **[MISSING]**
+- [ ] `getStarred` **[MISSING]**
+- [ ] `getStarred2` **[MISSING]**
+
+#### Searching
+- [x] `search` (Will never be implemented, deprecated)
+- [ ] `search2` **[MISSING]**
+- [x] `search3`
+
+#### Playlists
+- [x] `getPlaylists`
+- [x] `getPlaylist`
+- [x] `createPlaylist`
+- [x] `updatePlaylist`
+- [x] `deletePlaylist`
+
+#### Media retrieval
+- [x] `stream` *(Only URL handling)*
+- [x] `download`
+- [x] `hls` *(Only URL handling)*
+- [x] `getCaptions`
+- [x] `getCoverArt`
+- [ ] `getLyrics` **[MISSING]**
+- [x] `getAvatar`
+
+#### Media annotation
+- [x] `star`
+- [x] `unstar`
+- [x] `setRating`
+- [x] `scrobble`
+
+#### Sharing
+- [x] `getShares`
+- [x] `createShare`
+- [x] `updateShare`
+- [x] `deleteShare`
+
+#### Podcast
+- [x] `getPodcasts`
+- [x] `getNewestPodcasts`
+- [x] `refreshPodcasts`
+- [x] `createPodcastChannel`
+- [x] `deletePodcastChannel`
+- [x] `deletePodcastEpisode`
+- [x] `downloadPodcastEpisode`
+
+#### Jukebox
+- [x] `jukeboxControl`
+
+#### Internet radio
+- [x] `getInternetRadioStations`
+- [x] `createInternetRadioStation`
+- [x] `updateInternetRadioStation`
+- [x] `deleteInternetRadioStation`
+
+#### Chat
+- [x] `getChatMessages`
+- [x] `addChatMessage`
+
+#### User management
+- [x] `getUser`
+- [x] `getUsers`
+- [x] `createUser`
+- [x] `updateUser`
+- [x] `deleteUser`
+- [x] `changePassword`
+
+#### Bookmarks
+- [x] `getBookmarks`
+- [x] `createBookmark`
+- [x] `deleteBookmark`
+- [x] `getPlayQueue`
+- [x] `savePlayQueue`
+
+#### Media library scanning
+- [x] `getScanStatus`
+- [x] `startScan`
+
+### Missing model properties
+#### Album
+- [ ] `recordLabels`
+- [ ] `musicBrainzId`
+- [ ] `genres`
+- [ ] `artists`
+- [ ] `displayArtist`
+- [ ] `releaseTypes`
+- [ ] `moods`
+- [ ] `sortName`
+- [ ] `originalReleaseDate`
+- [ ] `releaseDate`
+- [ ] `isCompilation`
+- [ ] `discTitles`
+
+#### Artist
+- [ ] `musicBrainzId`
+- [ ] `sortName`
+- [ ] `roles`
+
+#### Song
+- [ ] `bmp`
+- [ ] `comment`
+- [ ] `sortName`
+- [ ] `musicBrainzId`
+- [ ] `genres`
+- [ ] `artists`
+- [ ] `displayArtist`
+- [ ] `albumArtists`
+- [ ] `displayAlbumArtist`
+- [ ] `contributors`
+- [ ] `displayComposer`
+- [ ] `moods`
+- [ ] `replayGain`