Add text about extensions

Mimic opencontainers/image-spec#164
and they should be kept in-sync

Signed-off-by: Doug Davis <dug@us.ibm.com>

config.md

@@ -308,7 +308,10 @@ The semantics are the same as `Path`, `Args` and `Env` in [golang Cmd](https://g
 
 This OPTIONAL property contains arbitrary metadata for the container.
 This information MAY be structured or unstructured.
-Annotations are key-value maps.
+Annotations MUST be key-value maps.
+Keys MUST be unique, and best practice is to namespace the keys.
+If there are no annotations then this property MAY either be absent or an empty map.
+Implementations that are reading/processing this configuration file MUST NOT generate an error if they encounter an unknown annotation.
 
 ```json
 "annotations": {
@@ -317,6 +320,10 @@ Annotations are key-value maps.
 }
 ```
 
+## Extensibility
+The `annotations` property MAY be used as an extensibility point to include additional information that is not defined as part of this specification.
+Excluding the `annotations` property, implementations that are reading/processing this configuration file MUST generate an error if they encounter an unkown property.
+
 ## Configuration Schema Example
 
 Here is a full example `config.json` for reference.