descriptor: add OPTIONAL annotations to this object

this has come up in discussion and is needed for adding discoverable
arbitrary data to the descriptor references. Especially in making an
index for image layout.

Signed-off-by: Vincent Batts <[email protected]>

Author: vbatts

descriptor.md

@@ -40,6 +40,11 @@ The following fields contain the primary properties that constitute a Descriptor
   Each entry MUST conform to [RFC 3986][rfc3986].
   Entries SHOULD use the `http` and `https` schemes, as defined in [RFC 7230][rfc7230-s2.7].
 
+- **`annotations`** *string-string map*
+
+    This OPTIONAL property contains arbitrary metadata for this descriptor.
+    This OPTIONAL property MUST use the [annotation rules](annotations.md#rules).
+
 ### Reserved
 
 The following field keys are reserved and MUST NOT be used by other specifications.