Release notes for 1.0.0.

Move the release notes document into the docs directory, and convert to
Markdown so that it can be rendered on the project's website.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/docs/index.md b/docs/index.md
index ca08c72..567f188 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -16,6 +16,7 @@
 ## Contents
 
 - General: this document
+- [Release notes]({% link release-notes.md %})
 - [design]({% link design.txt %}): for the design
 - [imgtool]({% link imgtool.md %}): The image signing and key management
   tool.