Move Kubernetes docs out of README

doc/kubernetes.md

@@ -0,0 +1,32 @@
+# Kubernetes / GKE
+
+This Gitea instance is currently hosted on Google Kubernetes Engine.
+
+## Apply changes to resources
+
+```
+kubectl apply -f gitea-db.yaml
+kubectl apply -f gitea-server.yaml
+```
+
+## Write the secrets to the local filesystem
+
+```
+./script/get_secrets
+```
+
+It writes the secrets (currently the app.ini file, as well as auto-generated
+TLS certificates that are only used when no Let's Encrypt cert is available)
+to the `kubernetes/config/` folder. These files are not in Git because they
+contain credentials.
+
+Once you have edited them locally, you need to delete the secrets stored on
+Kubernetes before uploading them again. This is done by this script:
+
+```
+./script/replace_secrets
+```
+
+## Reuse a released persistent volume:
+
+https://github.com/kubernetes/kubernetes/issues/48609#issuecomment-314066616