[DOCS] Add 6.8.4 release notes (#1377)

Author: jrodewig

docs/src/reference/asciidoc/appendix/release-notes/6.8.4.adoc

@@ -0,0 +1,6 @@
+[[eshadoop-6.8.4]]
+== Elasticsearch for Apache Hadoop version 6.8.4
+October 23, 2019
+
+ES-Hadoop 6.8.4 is a version compatibility release,
+tested specifically against Elasticsearch 6.8.4.

docs/src/reference/asciidoc/appendix/release.adoc

@@ -8,6 +8,7 @@ This section summarizes the changes in each release.
 [float]
 [[release-notes-6]]
 ===== 6.x
+* <<eshadoop-6.8.4>>
 * <<eshadoop-6.8.3>>
 * <<eshadoop-6.8.2>>
 * <<eshadoop-6.8.1>>
@@ -96,6 +97,7 @@ http://github.com/elastic/elasticsearch-hadoop/issues/XXX[#XXX]
 
 ////////////////////////
 
+include::release-notes/6.8.4.adoc[]
 include::release-notes/6.8.3.adoc[]
 include::release-notes/6.8.2.adoc[]
 include::release-notes/6.8.1.adoc[]