Add documentation for `Apply For` services

doc/15-Service-apply-for-example.md

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+<a id="Service-apply-for-example"></a>Working with Apply for rules - tcp ports example
+==============================================
+
+This example wants to show you how to make use of `Apply For` rule for services.
+
+First you need to define a `tcp_ports` data field of type `Array` assigned to a `Host Template`. 
+Refer to [Working with fields](14-Fields-example-interfaces-array.md) section to setup a data field.
+You also need to define a `tcp_port` data field of type `String`, we will associate it to a
+`Service Template` later.
+
+Then, please got to the `Dashboard` and choose the `Monitored services` dashlet:
+
+![Dashboard - Monitored services](screenshot/director/15_apply-for-services/151_monitored_services.png)
+
+Then create a new `Service template` with check command `tcp`:
+
+![Define service template - tcp](screenshot/director/15_apply-for-services/152_add_service_template.png)
+
+Then associate the data field `tcp_port` to this `Service template`:
+
+![Associate field to service template - tcp_port](screenshot/director/15_apply-for-services/153_add_service_template_field.png)
+
+Then create a new `apply-rule` for the `Service template`:
+
+![Define apply rule](screenshot/director/15_apply-for-services/154_create_apply_rule.png)
+
+Now define the `Apply For` property, select the previously defined field `tcp_ports` associated to
+the host template. `Apply For` rule define a variable `value` that can be used as `$$value$$`, it 
+corresponds to the item of the array it will iterate on. Set the `Tcp port` property to `$$value$$`:
+
+![Add field to template](screenshot/director/15_apply-for-services/155_configure_apply_for.png)
+
+(Side note: if you can't see your `tcp_ports` property in `Apply For` dropdown, try to create one 
+host with a non-empty `tcp_ports` value.)
+
+That's it, now all your hosts defining a `tcp_ports` variable will be assigned the `Tcp Check`
+service.
+
+Have a look at the config preview, it will show you how `Apply For` services will look like once 
+deployed:
+
+![Host config preview with Array](screenshot/director/15_apply-for-services/156_config_preview.png)