[DOCS] Add 7.3.1 release notes (#1335)

Author: jrodewig

docs/src/reference/asciidoc/appendix/release-notes/7.3.1.adoc

@@ -0,0 +1,5 @@
+[[eshadoop-7.3.1]]
+== Elasticsearch for Apache Hadoop version 7.3.1
+August 22, 2019
+
+ES-Hadoop 7.3.1 is a version compatibility release, tested specifically against Elasticsearch 7.3.1.

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

@@ -8,6 +8,7 @@ This section summarizes the changes in each release.
 [float]
 [[release-notes-7]]
 ===== 7.x
+* <<eshadoop-7.3.1>>
 * <<eshadoop-7.3.0>>
 * <<eshadoop-7.2.1>>
 * <<eshadoop-7.2.0>>
@@ -69,6 +70,7 @@ http://github.com/elastic/elasticsearch-hadoop/issues/XXX[#XXX]
 
 ////////////////////////
 
+include::release-notes/7.3.1.adoc[]
 include::release-notes/7.3.0.adoc[]
 include::release-notes/7.2.1.adoc[]
 include::release-notes/7.2.0.adoc[]