Add concept page for Operators (#14458)

* Add concept page for Operator pattern

* Link to article about Operator best practices

content/en/docs/reference/glossary/operator-pattern.md

@@ -0,0 +1,24 @@
+---
+title: Operator pattern
+id: operator-pattern
+date: 2019-05-21
+full_link: /docs/concepts/extend-kubernetes/operator/
+short_description: >
+  A specialized controller used to manage a custom resource
+
+aka:
+tags:
+- architecture
+---
+ The [operator pattern](/docs/concepts/extend-kubernetes/operator/) is a system
+design that links a {{< glossary_tooltip term_id="controller" >}} to one or more custom
+resources.
+
+<!--more-->
+
+You can extend Kubernetes by adding controllers to your cluster, beyond the built-in
+controllers that come as part of Kubernetes itself.
+
+If a running application acts as a controller and has API access to carry out tasks
+against a custom resource that's defined in the control plane, that's an example of
+the Operator pattern.