[DOCS] Add 6.8.15 release notes (#1623)

Author: jrodewig

docs/src/reference/asciidoc/appendix/release-notes/6.8.15.adoc

@@ -0,0 +1,7 @@
+[[eshadoop-6.8.15]]
+== Elasticsearch for Apache Hadoop version 6.8.15
+
+coming::[6.8.15]
+
+ES-Hadoop 6.8.15 is a version compatibility release,
+tested specifically against Elasticsearch 6.8.15.

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-6.8.15>>
 * <<eshadoop-6.8.14>>
 * <<eshadoop-6.8.13>>
 * <<eshadoop-6.8.12>>
@@ -108,6 +109,7 @@ http://github.com/elastic/elasticsearch-hadoop/issues/XXX[#XXX]
 
 ////////////////////////
 
+include::release-notes/6.8.15.adoc[]
 include::release-notes/6.8.14.adoc[]
 include::release-notes/6.8.13.adoc[]
 include::release-notes/6.8.12.adoc[]