[DOCS] Add 6.8.21 release notes (#1819)

Author: jrodewig

docs/src/reference/asciidoc/appendix/release-notes/6.8.21.adoc

@@ -0,0 +1,5 @@
+[[eshadoop-6.8.21]]
+== Elasticsearch for Apache Hadoop version 6.8.21
+
+ES-Hadoop 6.8.21 is a version compatibility release,
+tested specifically against Elasticsearch 6.8.21.

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

@@ -9,6 +9,7 @@ This section summarizes the changes in each release.
 [[release-notes-6]]
 ===== 6.x
 
+* <<eshadoop-6.8.21>>
 * <<eshadoop-6.8.20>>
 * <<eshadoop-6.8.19>>
 * <<eshadoop-6.8.18>>
@@ -114,6 +115,7 @@ http://github.com/elastic/elasticsearch-hadoop/issues/XXX[#XXX]
 
 ////////////////////////
 
+include::release-notes/6.8.21.adoc[]
 include::release-notes/6.8.20.adoc[]
 include::release-notes/6.8.19.adoc[]
 include::release-notes/6.8.18.adoc[]