Add instructions for setting localstack bucket CORS

CONTRIBUTING.md

@@ -48,6 +48,7 @@ And initialize the artifact bucket:
 
 ```
 aws s3api create-bucket --bucket artifacts
+aws s3api put-bucket-cors --bucket artifacts --cors-configuration file://local-cors-config.json
 ```
 
 #### Admin Panel

local-cors-config.json

@@ -0,0 +1,13 @@
+{
+  "CORSRules": [
+    {
+      "AllowedHeaders": ["*"],
+      "AllowedMethods": ["GET", "POST", "PUT", "HEAD", "DELETE"],
+      "AllowedOrigins": [
+        "http://127.0.0.1:3000",
+        "http://127.0.0.1:8080"
+      ],
+      "ExposeHeaders": ["ETag"]
+    }
+  ]
+}