Add release notes

Signed-off-by: Fabio Utzig <utzig@apache.org>

release-notes.rst

@@ -1,19 +1,32 @@
 mcuboot 0.9 - Release Notes
 ===========================
 
-Welcome to the first release of MCUBoot, a secure bootloader for 32-bit MCUs.
+This is the first release of MCUBoot, a secure bootloader for 32-bit MCUs.
 It is designed to be operating system-agnostic and works over any transport -
 wired or wireless. It is also hardware independent, and relies  on hardware
 porting layers from the operating system it works with. For the first release,
-we are pleased to announce support for three open source operating systems -
-Apache Mynewt, Zephyr Project, and RIOT.
+we have support for three open source operating systems: Apache Mynewt, Zephyr
+and RIOT.
 
 About this release:
 ===================
 
-* TODO
+* This release supports building with and running Apache Mynewt and Zephyr
+  targets.
+* RIOT is supported as a running target.
+* Image integrity is provided with SHA256.
+* Image originator authenticity is provided supporting the following
+  signature algorithms:
+  - RSA 2048 and RSA PKCS#1 v1.5 or v2.1
+  - Elliptic curve DSA with secp224r1 and secp256r1
+* Two firmware upgrade algorithms are provided:
+  - An overwrite only which upgrades slot 0 with the image in slot 1.
+  - A swapping upgrade which enables image test, allowing for rollback to a
+  previous known good image.
+* Supports both mbed-TLS and tinycrypt as backend crypto libraries.
 
 Known issues:
 =============
 
-* TODO
+* The image header and TLV formats are planned to change with release 1.0:
+  https://runtimeco.atlassian.net/browse/MCUB-66