Add encrypt images design/usage documentation

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/docs/design.md b/docs/design.md
index 5bd0b6a..4da4c99 100644
--- a/docs/design.md
+++ b/docs/design.md
@@ -685,3 +685,6 @@
 
 For information on embedding public keys in the boot loader, as well as
 producing signed images, see: [signed_images](signed_images.md).
+
+If you want to enable and use encrypted images, see:
+[encrypted_images](encrypted_images.md).