diff --git a/README.md b/README.md
index 3f8d432..08cf3e4 100644
--- a/README.md
+++ b/README.md
@@ -70,6 +70,8 @@ I have tested with CYGWIN without any problem, and I believe it also works on Mi
  * Install poppler-data if your want CJK support
 * fontforge (with header files)
  * git version is recommended to avoid annoying compilation issues
+* [Optional] ttfautohint
+ * use --external-hint-tool=ttfautohint to enable it
 
 #### Compiling