Explicitly opt out of Renovate (#31)

This repository doesn't gain any value from using Renovate, so we can be
super clear that we want to opt out.

Author: jamietanna

renovate.json

@@ -0,0 +1,4 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "enabled": false
+}