Add a list of categories

Author: Jon Staab

0a.md

@@ -34,6 +34,20 @@ The following tags are optional:
 - `duration` is the duration of the audio track, in seconds.
 - `published_at` is a timestamp representing the track's original publish date.
 
+Expected categories may include (but are not limited to):
+
+- `genre`
+- `subgenre`
+- `artist`
+- `record_label`
+- `producer`
+- `composer`
+- `album`
+- `track_number`
+- `tracks`
+- `bpm`
+- `year`
+
 Note that artists, producers, SHOULD be referred to using both a plaintext `c` tag AND a conventional `p` tag when pubkey is available. When doing so, the `p` tag's petname MUST be the same as the `c` tag's value. In this way `c` tags can be used to indicate genre, as well as artist, producer, etc.
 
 If included, `genre` and `subgenre` SHOULD be based on [this list](https://github.com/wavlake/genre-list).