Add Logging to glossary (#13608)

Associated with #5993

content/en/docs/reference/glossary/logging.md

@@ -0,0 +1,18 @@
+---
+title: Logging
+id: logging
+date: 2019-04-04
+full_link: /docs/concepts/cluster-administration/logging/
+short_description: >
+  Logs are the list of events that are logged by cluster or application.
+
+aka: 
+tags:
+- architecture
+- fundamental
+---
+ Logs are the list of events that are logged by {{< glossary_tooltip text="cluster" term_id="cluster" >}} or application.
+
+<!--more--> 
+
+Application and systems logs can help you understand what is happening inside your cluster. The logs are particularly useful for debugging problems and monitoring cluster activity.