Add changelogUrl to Core manifest.

Core.json

@@ -11,6 +11,7 @@
   "manualUrl": "https://vcvrack.com/manual/Core",
   "sourceUrl": "https://github.com/VCVRack/Rack",
   "donateUrl": "",
+  "changelogUrl": "https://github.com/VCVRack/Rack/blob/v2/CHANGELOG.md",
   "description": "Necessary modules built into VCV Rack",
   "modules": [
     {