docs: add encryption key management and recovery guide to README

- Recommend explicit encryptionKey setting at install time
- Add kubectl recovery procedure for key mismatch (works even when
  pod is in CrashLoopBackOff via a temporary reader pod)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Chart.yaml

@@ -2,7 +2,7 @@ apiVersion: v2
 name: n8n
 description: A Helm chart for n8n workflow automation on Kubernetes (ARM/Raspberry Pi ready)
 type: application
-version: "2.19.2-b"
+version: "2.19.2-c"
 appVersion: "2.19.2"
 keywords:
   - n8n