Card: describe how samples were chosen

README.md

@@ -37,7 +37,7 @@ sums the record publishes (`annotations.json`
 
 ## Samples
 
-Four random test images (fixed seed) with the boxes of every annotated subject and object; an object annotated twice with slightly different boxes appears twice. Green lines are relations labelled true, red lines relations labelled false.
+Four test images drawn at random (fixed seed) from the 22 that have at least three relations, both labels and at most five objects, with the boxes of every annotated subject and object; an object annotated twice with slightly different boxes appears twice. Green lines are relations labelled true, red lines relations labelled false.
 
 <p>
 <img src="https://huggingface.co/datasets/maelic/SpatialSense-coco-format/resolve/main/assets/sample_1.jpg" width="49%">