Preps for 1.6.0-rc1

Update version fields, and add release notes for the 1.6.0-rc1 release.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/README.md b/README.md
index b5018cc..9ff247a 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@
 [coverity]: https://scan.coverity.com/projects/mcuboot
 [travis]: https://travis-ci.org/JuulLabs-OSS/mcuboot
 
-This is mcuboot version 1.5.0
+This is mcuboot version 1.6.0-rc1
 
 MCUboot is a secure bootloader for 32-bit MCUs. The goal of MCUboot is to
 define a common infrastructure for the bootloader, system flash layout on