Update docs/METADATA.md to reflect recent spec change

The specification was updated in PR #40 to remove root.json from
snapshot.json

Signed-off-by: Joshua Lock <jlock@vmware.com>

docs/METADATA.md

@@ -64,7 +64,7 @@ of delegated Targets metadata and [example](https://raw.githubusercontent.com/th
 
 Signed by: Snapshot role.
 
-The snapshot.json metadata file lists hashes and sizes of all metadata files other than timestamp.json. This file ensures that clients will see a consistent view of the files on the repository. That is, metadata files (and thus target file) that existed on the repository at different times cannot be combined and presented to clients by an attacker.
+The snapshot.json metadata file lists the version, and optionally the file hashes and sizes, of all metadata files other than root.json and timestamp.json. This file ensures that clients will see a consistent view of the files on the repository. That is, metadata files (and thus target file) that existed on the repository at different times cannot be combined and presented to clients by an attacker.
 
 ​See [example](https://raw.githubusercontent.com/theupdateframework/tuf/develop/tests/repository_data/repository/metadata/snapshot.json) of Snapshot metadata.