Update README.md

README.md

@@ -21,6 +21,7 @@ The images are intended to be used for different AI/ML tasks such as generating
 The images themselves are JPEG files under the directory `images`.
 
 The `metadata.jsonl` file contains metadata about each image, for example descriptive captions (mostly in Finnish), subject keywords, location and the name of the photographer.
+The file is in JSON Lines format (each line is a JSON record). The `file_name` column contains the relative path of the JPEG image file.
 
 The Python script `collect-hkm.py` was originally used to retrieve the metadata and images from the from Finna API.