Convert design document to Markdown

The complex blocks are just verbatim, and some of the outline-style
blocks are keps as pre formatted until we decide the right way to format
them.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/docs/index.md b/docs/index.md
index c780477..41befd7 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -17,7 +17,7 @@
 
 - General: this document
 - [Release notes]({% link release-notes.md %})
-- [design]({% link design.txt %}): for the design
+- [design]({% link design.md %}): for the design
 - [imgtool]({% link imgtool.md %}): The image signing and key management
   tool.
 - [Patch submission]({% link SubmittingPatches.md %}) for information