Specification of how Mbed Crypto uses storage

Describe the storage format for keys and random seed in Mbed Crypto
0.1.0 released with Mbed OS 5.11, over C stdio and over ITS with
32-bit file identifiers.

Describe the proposed storage format for keys and random seed in the
future release of Mbed Crypto for Mbed OS 5.12, over C stdio and over
ITS with 64-bit file identifiers.
diff --git a/docs/architecture/Makefile b/docs/architecture/Makefile
index 4873dae..f763c9c 100644
--- a/docs/architecture/Makefile
+++ b/docs/architecture/Makefile
@@ -3,6 +3,7 @@
 default: all
 
 all_markdown = \
+	       mbed-crypto-storage-specification.md \
 	       # This line is intentionally left blank
 
 html: $(all_markdown:.md=.html)