Docs: Add glossary (#1024)

Author: csviri

docs/_data/sidebar.yml

@@ -5,6 +5,8 @@
     url: /docs/getting-started
   - title: How to use Samples
     url: /docs/using-samples
+  - title: Glossary
+    url: /docs/glossary
   - title: Features
     url: /docs/features
   - title: Patterns and Best Practices

docs/documentation/glossary.md

@@ -0,0 +1,17 @@
+---
+title: Glossary description: Glossary layout: docs permalink: /docs/glossary
+---
+
+# Glossary
+
+- Primary Resource - the resource that represents the desired state that the controller is working
+  to achieve. While this is often a Custom Resource, this can also be a Kubernetes native resource.
+- Secondary Resource - any resource that the controller needs to achieve the desired state
+  represented by the primary resource. These resources can be created, updated, deleted or simply
+  read depending on the use case. For example, the `Deployment` controller manages `ReplicatSet`
+  instances when trying to realize the state represented by the `Deployment`. In this scenario,
+  the `Deployment` is the primary resource while `ReplicaSet` is one of the secondary resources
+  managed by the `Deployment` controller.
+- Dependent Resource - a feature of JOSDK, aimed at making easier to manage secondary resources. A
+  dependent resource is therefore a secondary resource implemented using this specific JOSDK
+  feature.