Harbor decoder (#43)

* Fix link to builtins.

* Add 'decoder.harbor@v1.star' initial draft.

README.md

@@ -163,7 +163,7 @@ There are a few builtin decoders and post hooks, to support some common use
 cases. For example decoding events from a oci distribution registry, or opening
 a pull request on github.
 
-The starlark code for the builtins can be reviewed in the [builtin](./builtin)
+The starlark code for the builtins can be reviewed in the [builtin](./plugin/builtin)
 directory.
 
 #### Decoder

plugin/builtin/starlark/decoder.harbor@v1.star

@@ -0,0 +1,12 @@
+load("encoding/json.star", "json")
+
+def main(input):
+    data = json.decode(input)
+
+    event = data["event_data"]
+
+    output = []
+    for resource in event["resources"]:
+        output.append(resource["resource_url"])
+
+    return output