Add APM .NET agent reference (#946)

Author: bmorelli25

shared/attributes.asciidoc

@@ -36,6 +36,7 @@
 :apm-ruby-ref:         https://www.elastic.co/guide/en/apm/agent/ruby/current
 :apm-java-ref:         https://www.elastic.co/guide/en/apm/agent/java/current
 :apm-go-ref:           https://www.elastic.co/guide/en/apm/agent/go/current
+:apm-dotnet-ref:       https://www.elastic.co/guide/en/apm/agent/dotnet/current
 :hadoop-ref:           https://www.elastic.co/guide/en/elasticsearch/hadoop/{branch}
 :stack-ref:            http://www.elastic.co/guide/en/elastic-stack/{branch}
 :stack-ref-67:         http://www.elastic.co/guide/en/elastic-stack/6.7
@@ -150,4 +151,4 @@
 :api-query-parms-title:    Query parameters
 :api-request-body-title:   Request body
 :api-response-body-title:  Response body
-:api-example-title:        Example
+:api-example-title:        Example