Update README.md

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: L-M-Sherlock

README.md

@@ -74,6 +74,7 @@ for (date, rating) in history {
 }
 
 let parameters = compute_parameters(ComputeParametersInput {
+    // For best results, `train_set` should contain review histories from many cards.
     train_set: items,
     ..Default::default()
 })?;