serialization: add language improvement to serialization

A small fix from the v1.1 merge that happened inside of the docker repo
after we pulled this in.

See moby/moby#22264

Signed-off-by: Brandon Philips <brandon.philips@coreos.com>

serialization.md

@@ -507,6 +507,11 @@ For example, here's what the full archive of `library/busybox` is (displayed in
 |-- repositories
 ```
 
+There is a directory for each layer in the image.
+Each directory is named with 64 character hex name that is deterministically generated from the layer information.
+These names are not necessarily layer DiffIDs or ChainIDs.
+Each of these directories contains 3 files:
+
 There are one or more directories named with the ChainID for each layer in a full image.
 Each of these directories contains 3 files: