Add ecs to terminology list (#254)

* Add ecs to terminology list

* Modify ecs definition

Author: karenzone

docs/en/glossary/glossary.asciidoc

@@ -235,6 +235,14 @@ getting or searching for a document.
 +
 //Source: Elasticsearch
 endif::elasticsearch-terms[]
+
+[[glossary-ecs]] Elastic Common Schema (ECS) ::
+
+A document schema for Elasticsearch, for use cases such as logging and metrics.
+ECS defines a common set of fields, their datatype, and gives guidance on their
+correct usage. ECS is used to improve uniformity of event data coming from
+different sources.
+
 ifdef::logstash-terms[]
 
 [[glossary-event]] event ::