DOC: Add initial custom resource documentation

Author: Mo3m3n

documentation/custom-resources.md

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+# Custom Resources
+
+- In order to use custom resources, you will need to apply/update resource [definitions](../crs/definition/)
+- Custom Resources are used by Ingress Controller to implement HAProxy concepts like (backend, frontend, http rules, etc) which are all available under the `core.haproxy.org` API.
+- Current implementation relies on the [client-native](https://github.com/haproxytech/client-native) library and its [models](https://github.com/haproxytech/client-native/tree/master/models) to [configure HAProxy](https://cbonte.github.io/haproxy-dconv/2.4/configuration.html#4.1).
+- Custom resources are meant to **replace annotations** when possible. So they will have **precedance** when used.   
+  *Example:* if the backend resource is used no backend annotation will be processed which means a backend cannot be configured by mixing both the backend resource and backend annotations.
+
+## HAProxy concepts
+- Only HAProxy directives available in the resource [definitions](../crs/definition/) are supported, contributions and github requests to support new directives are welcome.
+- All timeout fields are integer input interpreted as time in **ms**.
+
+### Global
+The Global resource is used to configure the HAProxy global section by referencing the resouce via the `cr-global` annotation in the Ingress Controller ConfigMap.
+
+*Example:*
+
+1. Define a global resource
+```yaml
+apiVersion: "core.haproxy.org/v1alpha1"
+kind: Global
+metadata:
+  name: myglobal
+  namespace: haproxy-controller
+spec:
+  config:
+    maxconn: 1000
+    stats_timeout: 36000
+    tune_ssl_default_dh_param: 2048
+    ssl_default_bind_options: "no-sslv3 no-tls-tickets no-tlsv10"
+    ssl_default_bind_ciphers:  ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!3DES:!MD5:!PSK
+    hard_stop_after: 30000
+    server_state_base: /tmp/haproxy-ingress/state
+    runtime_apis:
+      - address: "0.0.0.0:31024"
+```
+
+2. Apply it:
+```
+$ kubectl apply -f myglobal.yaml
+```
+
+3. Update the ConfigMap
+```
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: kubernetes-ingress
+  namespace: haproxy-controller
+data:
+  cr-global: haproxy-controller/myglobal
+```
+
+### Defaults
+The Defaults resource is used to configure the HAProxy defaults section by referencing the resouce via the `cr-defaults` annotation in the Ingress Controller ConfigMap.
+
+*Example:*
+
+1. Define a defaults resource
+```yaml
+apiVersion: "core.haproxy.org/v1alpha1"
+kind: Defaults
+metadata:
+  name: mydefaults
+  namespace: default
+spec:
+  config:
+    log_format: "'%ci:%cp [%tr] %ft %b/%s %TR/%Tw/%Tc/%Tr/%Ta %ST %B %CC %CS %tsc %ac/%fc/%bc/%sc/%rc %sq/%bq %hr %hs \"%HM %[var(txn.base)] %HV\"'"
+    redispatch:
+      enabled: enabled
+      interval: 0
+    dontlognull: enabled
+    http_connection_mode: http-keep-alive
+    http_request_timeout: 5000
+    connect_timeout: 5000
+    client_timeout: 50000
+    queue_timeout: 5000
+    server_timeout: 50000
+    tunnel_timeout: 3600000
+    http_keep_alive_timeout: 60000
+```
+
+2. Apply it:
+```
+$ kubectl apply -f mydefaults.yml
+```
+
+3. Update the ConfigMap
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: kubernetes-ingress
+  namespace: haproxy-controller
+data:
+  cr-global: haproxy-controller/myglobal
+  cr-defaults: haproxy-controller/mydefaults
+```
+
+
+### Backend
+The Backend resource is used to configure the HAProxy backend section by referencing the resouce via the `cr-backend` annotation in corresponding backend service.   
+`cr-backend` annotation can be used also at the ConfigMap level (as default backend config for all services) or Ingress level (as a default backend config for the underlying services)
+
+*Example:*
+
+1. Define a backend resource
+```yaml
+apiVersion: "core.haproxy.org/v1alpha1"
+kind: Backend
+metadata:
+  name: mybackend
+  namespace: haproxy-controller
+spec:
+  config:
+    mode: http
+    balance:
+      algorithm: "leastconn"
+    abortonclose: disabled
+    default_server:
+      verify: none
+      resolve-prefer: ipv4
+      check-sni: example.com
+      sni: str(example.com)
+```
+
+2. Apply it:
+```
+$ kubectl apply -f mybackend.yaml
+```
+
+3. Annotate the corresponding service
+```yaml
+apiVersion: v1
+kind: Service
+metadata:
+  name: example
+  namespace: external
+  annotations:
+    cr-backend: haproxy-controller/mybackend
+spec:
+  type: ExternalName
+  externalName: example.com
+  ports:
+  - protocol: TCP
+    port: 443
+    name: https
+    targetPort: 443
+```