Add docs for pdns provider

docs/tutorials/pdns.md

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+# Setting up ExternalDNS for PowerDNS
+
+## Prerequisites
+
+The provider has been written for and tested against [PowerDNS](https://github.com/PowerDNS/pdns) v4.1.x and thus requires **PowerDNS Auth Server >= 4.1.x**
+
+PowerDNS provider support was added via [this PR](https://github.com/kubernetes-incubator/external-dns/pull/373), thus you need to use external-dns version >= v0.5
+
+The PDNS provider expects that your PowerDNS instance is already setup and
+functional. It expects that zones, you wish to add records to, already exist
+and are configured correctly. It does not add, remove or configure new zones in
+anyway.
+
+## Feature Support
+
+The PDNS provider currently does not support:
+
+1. Dry running a configuration is not supported.
+2. The `--domain-filter` flag is not supported.
+
+## Deployment
+
+Deploying external DNS for PowerDNS is actually nearly identical to deploying
+it for other providers. This is what a sample `deployment.yaml` looks like:
+
+```yaml
+apiVersion: extensions/v1beta1
+kind: Deployment
+metadata:
+  name: deploy-external-dns
+spec:
+  strategy:
+    type: Recreate
+  template:
+    metadata:
+      labels:
+        app: external-dns
+    spec:
+      # Only use if you're also using RBAC
+      # serviceAccountName: external-dns
+      containers:
+      - name: external-dns
+        image: registry.opensource.zalan.do/teapot/external-dns:v0.4.8
+        args:
+        - --source=service # or ingress or both
+        - --provider=pdns
+        - --pdns-server={{ pdns-api-url }}
+        - --pdns-api-key={{ pdns-http-api-key }}
+        - --txt-owner-id={{ owner-id-for-this-external-dns }}
+        - --log-level=debug
+        - --interval=30s
+```
+
+## RBAC
+
+If your cluster is RBAC enabled, you also need to setup the following, before you can run external-dns:
+```yaml
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: external-dns
+---
+apiVersion: rbac.authorization.k8s.io/v1beta1
+kind: ClusterRole
+metadata:
+  name: external-dns
+rules:
+- apiGroups: [""]
+  resources: ["services"]
+  verbs: ["get","watch","list"]
+- apiGroups: ["extensions"]
+  resources: ["ingresses"]
+  verbs: ["get","watch","list"]
+- apiGroups: [""]
+  resources: ["pods"]
+  verbs: ["get","watch","list"]
+---
+apiVersion: rbac.authorization.k8s.io/v1beta1
+kind: ClusterRoleBinding
+metadata:
+  name: external-dns-viewer
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: external-dns
+subjects:
+- kind: ServiceAccount
+  name: external-dns
+  namespace: default
+```
+
+## Testing and Verification
+
+**Important!**: Remember to change `example.com` with your own domain throughout the following text.
+
+Spin up a simple "Hello World" HTTP server with the following spec (`kubectl apply -f`):
+
+```yaml
+apiVersion: extensions/v1beta1
+kind: Deployment
+metadata:
+  name: echo
+spec:
+  template:
+    metadata:
+      labels:
+        app: echo
+    spec:
+      containers:
+      - image: hashicorp/http-echo
+        name: echo
+        ports:
+        - containerPort: 5678
+        args:
+          - -text="Hello World"
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: echo
+  annotations:
+    external-dns.alpha.kubernetes.io/hostname: echo.example.com
+spec:
+  selector:
+    app: echo
+  type: LoadBalancer
+  ports:
+    - protocol: TCP
+      port: 80
+      targetPort: 5678
+```
+**Important!**: Don't run dig, nslookup or similar immediately (until you've
+confirmed the record exists). You'll get hit by [negative DNS caching](https://tools.ietf.org/html/rfc2308), which is hard to flush.
+
+Run the following to make sure everything is in order:
+
+```bash
+$ kubectl get services echo
+$ kubectl get endpoints echo
+```
+
+Make sure everything looks correct, i.e the service is defined and recieves a
+public IP, and that the endpoint also has a pod IP.
+
+Once that's done, wait about 30s-1m (interval for external-dns to kick in), then do:
+```bash
+$ curl -H "X-API-Key: ${PDNS_API_KEY}" ${PDNS_API_URL}/api/v1/servers/localhost/zones/example.com. | jq '.rrsets[] | select(.name | contains("echo"))'
+```
+
+Once the API shows the record correctly, you can double check your record using:
+```bash
+$ dig @${PDNS_FQDN} echo.example.com.
+```
+