canonicalization: Add recommendations for canonicalization

Spun off from [1], now that we seem to have reached a consensus for
"SHOULD canonical JSON" there.  I've set this up so we have space to
add canonicalization recommendations for other formats, although the
only other basic type discussed in this repository is a gzipped
tarball and that's more than I want to bite off at the moment ;).

[1]: opencontainers#259
     Subject: manifest json fields order

Signed-off-by: W. Trevor King <wking@tremily.us>

Makefile

@@ -30,7 +30,8 @@ DOC_FILES := \
 	layer.md \
 	config.md \
 	manifest.md \
-	manifest-list.md
+	manifest-list.md \
+	canonicalization.md
 
 FIGURE_FILES := \
 	img/media-types.png

README.md

@@ -14,6 +14,7 @@ The OCI Image Format project creates and maintains the software shipping contain
 - [Image Configuration](config.md)
 - [Image Manifest](manifest.md)
 - [Image Manifest List](manifest-list.md)
+- [Canonicalization](canonicalization.md)
 
 ## Overview
 

canonicalization.md

@@ -0,0 +1,21 @@
+# Canonicalization
+
+OCI Images [are](descriptor.md) [content-addressable](image-layout.md).
+One benefit of content-addressable storage is easy deduplication.
+Many images might depend on a particular [layer](layer.md), but there will only be one blob in the [store](image-layout.md).
+With a different serialization, that same semantic layer would have a different hash, and if both versions of the layer are referenced there will be two blobs with the same semantic content.
+To allow efficient storage, implementations serializing content for blobs SHOULD use a canonical serialization.
+This increases the chance that different implementations can push the same semantic content to the store without creating redundant blobs.
+
+## JSON
+
+[JSON][] content SHOULD be serialized as [canonical JSON][canonical-json].
+Of the [OCI Image Format Specification media types](media-types.md), all the types ending in `+json` contain JSON content.
+Implementations:
+
+* [Go][]: [github.com/docker/go][], which claims to implement [canonical JSON][canonical-json] except for Unicode normalization.
+
+[canonical-json]: http://wiki.laptop.org/go/Canonical_JSON
+[github.com/docker/go]: https://github.com/docker/go/
+[Go]: https://golang.org/
+[JSON]: http://json.org/