added reference to new arg 'subst' in the blurb in CHANGES.txt

Author: bdbaddog

CHANGES.txt

@@ -86,8 +86,9 @@ RELEASE  VERSION/DATE TO BE FILLED IN LATER
       updated in 4.6). All SCons usage except unit test was already fully
       consistent with a bool.
     - When a variable is added to a Variables object, it can now be flagged
-      as "don't perform substitution". This allows variables to contain
-      characters which would otherwise cause expansion. Fixes #4241.
+      as "don't perform substitution" by setting the argument subst. 
+      This allows variables to contain characters which would otherwise 
+      cause expansion. Fixes #4241.
     - The test runner now recognizes the unittest module's return code of 5,
       which means no tests were run. SCons/Script/MainTests.py currently
       has no tests, so this particular error code is expected - should not