Prepare for release v1.4.0-rc2

Update version stamps as well as the release notes.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/docs/release-notes.md b/docs/release-notes.md
index c42e7bc..1d0f842 100644
--- a/docs/release-notes.md
+++ b/docs/release-notes.md
@@ -17,11 +17,20 @@
 
 ### About this release
 
-- Fixed CVE-2019-5477.  This fixes an issue with a dependency used in
-  the generation of the documentation on github.
+- Fixed CVE-2019-5477, and CVE-2019-16892.  These fix issue with
+  dependencies used in the generation of the documentation on github.
 - Numerous code cleanups and refactorings
 - Documentation updates for multi-image features
 - Update imgtool.py to support the new features
+- Updated the mbed TLS submodule to current stable version 2.16.3
+- Moved the mbed TLS submodule from within sim/mcuboot-sys to ext.
+  This will make it easier for other board supports to use this code.
+- Added some additional overflow and bound checks to data in the image
+  header, and TLV data.
+- Add a `-x` (or `--hex_addr`) flag to imgtool to set the base address
+  written to a hex-format image.  This allows the image to be flashed
+  at an offset, without having to use additional tools to modify the
+  image.
 
 ## Version 1.3.1