doc: kubernetes CRD provider

Co-authored-by: Jean-Baptiste Doumenjou <jb.doumenjou@gmail.com>

docs/content/reference/providers/kubernetescrd.md

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+# Kubernetes -- Reference
+
+## Kubernetes
+
+```yaml
+################################################################
+# Kubernetes Provider
+################################################################
+
+apiVersion: apiextensions.k8s.io/v1beta1
+kind: CustomResourceDefinition
+metadata:
+  name: ingressroutes.traefik.containo.us
+
+spec:
+  group: traefik.containo.us
+  version: v1alpha1
+  names:
+    kind: IngressRoute
+    plural: ingressroutes
+    singular: ingressroute
+  scope: Namespaced
+
+---
+apiVersion: apiextensions.k8s.io/v1beta1
+kind: CustomResourceDefinition
+metadata:
+  name: middlewares.traefik.containo.us
+spec:
+  group: traefik.containo.us
+  version: v1alpha1
+  names:
+    kind: Middleware
+    plural: middlewares
+    singular: middleware
+  scope: Namespaced
+
+---
+apiVersion: traefik.containo.us/v1alpha1
+kind: IngressRoute
+metadata:
+  name: ingressroute.crd
+spec:
+  entrypoints:
+    - web
+    - web-secure
+  routes:
+  - match: Host(`foo.com`) && PathPrefix(`/bar`)
+    kind: Rule
+    priority: 12
+    # defining several services is possible and allowed, but for now the servers of
+    # all the services (for a given route) get merged altogether under the same
+    # load-balancing strategy.
+    services:
+    - name: s1
+      port: 80
+      healthcheck:
+        path: /health
+        host: baz.com
+        intervalseconds: 7
+        timeoutseconds: 60
+      # strategy defines the load balancing strategy between the servers. It defaults
+      # to Round Robin, and for now only Round Robin is supported anyway.
+      strategy: RoundRobin
+    - name: s2
+      port: 433
+      healthcheck:
+        path: /health
+        host: baz.com
+        intervalseconds: 7
+        timeoutseconds: 60
+  - match: PathPrefix(`/misc`)
+    services:
+    - name: s3
+      port: 80
+    middleware:
+    - name: stripprefix
+    - name: addprefix
+  tls:
+    secretName: supersecret
+```