diff --git a/README.md b/README.md
index 4154ee9..333b108 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@ describes the lanuage syntax
 
 The rest of the heavy lifting is done by a modified version of
 [ssg](https://www.romanzolotarev.com/ssg.html) which is included
-in this source code.
+in this source code. ssg requires a POSIX /bin/sh, and awk.
 
 ## TODOs