add coderabbit config (#2060)

Code Rabbit is one of these new fancy LLM code review tools. I am skeptical
but we can try it for free and it might provide us with some value to let
people get feedback while waiting for other people.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

.coderabbit.yaml

@@ -0,0 +1,15 @@
+# yaml-language-server: $schema=https://coderabbit.ai/integrations/schema.v2.json
+language: "en-GB"
+early_access: false
+reviews:
+  profile: "chill"
+  request_changes_workflow: false
+  high_level_summary: true
+  poem: true
+  review_status: true
+  collapse_walkthrough: false
+  auto_review:
+    enabled: true
+    drafts: true
+chat:
+  auto_reply: true