Add renovate config (#180)

The `renovate-bot` user must also be added to the repo to make this work.

(For automated onboarding, the user could also be added to the `gitea` org and given write permissions to all repos)

Reviewed-on: https://gitea.com/gitea/homebrew-gitea/pulls/180
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Co-committed-by: pat-s <patrick.schratz@gmail.com>

renovate.json5

@@ -0,0 +1,4 @@
+{
+  $schema: "https://docs.renovatebot.com/renovate-schema.json",
+  extends: ["gitea>gitea/renovate-config"],
+}