[DOCS] Add 6.8.8 release notes (#1448)

Author: jrodewig

docs/src/reference/asciidoc/appendix/release-notes/6.8.8.adoc

@@ -0,0 +1,5 @@
+[[eshadoop-6.8.8]]
+== Elasticsearch for Apache Hadoop version 6.8.8
+
+ES-Hadoop 6.8.8 is a version compatibility release,
+tested specifically against Elasticsearch 6.8.8.

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-6.8.8>>
 * <<eshadoop-6.8.7>>
 * <<eshadoop-6.8.6>>
 * <<eshadoop-6.8.5>>
@@ -101,6 +102,7 @@ http://github.com/elastic/elasticsearch-hadoop/issues/XXX[#XXX]
 
 ////////////////////////
 
+include::release-notes/6.8.8.adoc[]
 include::release-notes/6.8.7.adoc[]
 include::release-notes/6.8.6.adoc[]
 include::release-notes/6.8.5.adoc[]