Configure Renovate (#397)

* Add renovate.json

* Enable automerge for renovate

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: Tokuhiro Matsuno <tokuhirom@gmail.com>

Author: renovate[bot]

renovate.json

@@ -0,0 +1,8 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:base"
+  ],
+  "automerge": true,
+  "platformAutomerge": true
+}