Add ecs to terminology list

[karenzone]

Add Elastic Common Schema (ECS) to the glossary

[karenzone]

Moving from a slack thread:

@webmat suggested:

"The Elastic Common Schema (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 ingested into Elasticsearch."

I like where this is heading. For the glossary, we need to define what ECS is, not what it does.

[lcawl]

LGTM

[webmat]

Here's an attempt at what it actually is :-)

The Elastic Common Schema (ECS) is 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.

[webmat]

LGTM