Add attribute for ECS docs (#761)

Author: dedemorton

shared/attributes.asciidoc

@@ -54,6 +54,7 @@
 :curator-ref-current:  https://www.elastic.co/guide/en/elasticsearch/client/curator/current
 :infra-guide:          https://www.elastic.co/guide/en/infrastructure/guide/{branch}
 :sql-odbc:             https://www.elastic.co/guide/en/elasticsearch/sql-odbc/{branch}
+:ecs-ref:              https://www.elastic.co/guide/en/ecs/{branch}
 
 :forum:                https://discuss.elastic.co/
 :xpack-forum:          https://discuss.elastic.co/c/50-x-pack