[DOCS] Add 7.15 release notes (elastic#1773) (elastic#1775)

* [DOCS] Add 7.15 rns

* Add missing include::

Author: Adam Locke

docs/src/reference/asciidoc/appendix/release-notes/7.15.1.adoc

@@ -0,0 +1,7 @@
+[[eshadoop-7.15.1]]
+== Elasticsearch for Apache Hadoop version 7.15.1
+
+coming::[7.15.1]
+
+ES-Hadoop 7.15.1 is a version compatibility release, tested specifically against
+Elasticsearch 7.15.1.

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-7.15.1>>
 * <<eshadoop-7.15.0>>
 * <<eshadoop-7.14.2>>
 * <<eshadoop-7.14.1>>
@@ -106,6 +107,7 @@ http://github.com/elastic/elasticsearch-hadoop/issues/XXX[#XXX]
 
 ////////////////////////
 
+include::release-notes/7.15.1.adoc[]
 include::release-notes/7.15.0.adoc[]
 include::release-notes/7.14.2.adoc[]
 include::release-notes/7.14.1.adoc[]