feat(header): added headers, updated readme

README.md

@@ -56,6 +56,8 @@ An example of an entry from the dataset is given below:
 
 Each dataset entry therefore consists of the following fields:
 
+- `header` - main category of the text;
+- `subheader` - secondary category of the text (none in some cases);
 - `text` - text said by the champion or description of sound made by the champion;
 - `source` - link to the audio file;
 - `champion` - name of the champion in lowercase.