docs: Write up some docs about ecdsa padding

Start with some documentation on ECDSA signatures, and the problems with
the current padding approach.  Present a plan to support correctly
formatted ECDSA signatures, and how to handle the transition both in the
C code, as well as the tooling that signs images.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/docs/index.md b/docs/index.md
index d9c70cb..3c4d1a3 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -20,6 +20,7 @@
 - [Bootloader design](design.md)
 - [Encrypted images](encrypted_images.md)
 - [imgtool](imgtool.md) - image signing and key management
+- [ecdsa](ecdsa.md) - Information about ECDSA signature formats
 - Usage instructions:
   - [Zephyr](readme-zephyr.md)
   - [Mynewt](readme-mynewt.md)