added mention of Subst

--- a/README	Fri Dec 03 12:41:54 1993 +0100
+++ b/README	Mon Dec 13 18:43:03 1993 +0100
@@ -11,6 +11,8 @@
 ex -- subdirectory containing examples.  To execute them, enter an ML image
 containing HOL and type:    use "ex/ROOT.ML";
 
+Subst -- subdirectory defining a theory of substitution and unification.
+
 Useful references on Higher-Order Logic:
 
 	P. B. Andrews,