[DOCS] Add 7.8.0 release notes (elastic#1488) (elastic#1491)

Author: jrodewig

docs/src/reference/asciidoc/appendix/release-notes/7.8.0.adoc

@@ -0,0 +1,5 @@
+[[eshadoop-7.8.0]]
+== Elasticsearch for Apache Hadoop version 7.8.0
+
+ES-Hadoop 7.8.0 is a version compatibility release, tested specifically against 
+Elasticsearch 7.8.0.

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

@@ -9,6 +9,7 @@ This section summarizes the changes in each release.
 [[release-notes-7]]
 ===== 7.x
 
+* <<eshadoop-7.8.0>>
 * <<eshadoop-7.7.1>>
 * <<eshadoop-7.7.0>>
 * <<eshadoop-7.6.2>>
@@ -83,6 +84,7 @@ http://github.com/elastic/elasticsearch-hadoop/issues/XXX[#XXX]
 
 ////////////////////////
 
+include::release-notes/7.8.0.adoc[]
 include::release-notes/7.7.1.adoc[]
 include::release-notes/7.7.0.adoc[]
 include::release-notes/7.6.2.adoc[]