diff --git a/docs/src/reference.md b/docs/src/reference.md
index 341e1361..d3676fff 100644
--- a/docs/src/reference.md
+++ b/docs/src/reference.md
@@ -1,5 +1,7 @@
 # Reference
 
+We refer these repos to build `model` and `engine`:
+
 * [delta](https://github.com/Delta-ML/delta.git)
 * [espnet](https://github.com/espnet/espnet.git)
 * [kaldi](https://github.com/kaldi-asr/kaldi.git)