Update README documentation

Changes:
    - Added note that the "extended" directory contains legacy
      REST and will be deprecated and no longer supported.

Tested:
    - NA
Change-Id: Ia952a5d9dcc05c1465e2d7e667dcf3f8187b4037
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
diff --git a/README.md b/README.md
index 32a8446..67b9204 100644
--- a/README.md
+++ b/README.md
@@ -154,7 +154,8 @@
 at some point and therefore no longer supported.
 
 `extended/`: Contains test cases for boot testing, code update testing using
-legacy REST, etc.
+legacy REST, etc. The "extended" subdirectory uses legacy REST and will be
+deprecated at some point and therefore no longer supported.
 
 `systest/`: Contains test cases for HTX bootme testing.