Add optional image key

Co-authored-by: Simon Li <[email protected]>

spec.md

@@ -141,7 +141,26 @@ the value is an RGBA color (4 byte, `0-255` per channel) for representing the ob
     "1": 8388736,
     ...
 ```
+### "image"
 
+The `image` key is an optional dictionary which contains information on the image the mask is associated with.
+If included it must include a key `array` whose value that is either:
+- A relative path to a Zarr image array, for example:
+    ```
+    {
+      "image": {
+        "array": "../../0"
+      }
+    }
+    ```
+- A URL to a Zarr image array (use this if the mask is stored seperately from the image Zarr), for example:
+    ```
+    {
+      "image": {
+        "array": "https://s3.embassy.ebi.ac.uk/idr/zarr/v0.1/6001240.zarr/0"
+      }
+    }
+    ```