Add note to RELEASE.txt as well

Author: acmorrow

RELEASE.txt

@@ -63,6 +63,8 @@ IMPROVEMENTS
   the new scheduler is now used for -j1 builds as well.
   NOTE: This should significantly improve SCons performance for larger parallel builds
         (Larger -j values)
+- CacheDir writes no longer happen within the taskmaster critical section, and therefore
+  can run in parallel with both other CacheDir writes and the taskmaster DAG walk.
 
 
 PACKAGING