[DOCS] Add 7.4.0 release notes (#1356)

Author: jrodewig

docs/src/reference/asciidoc/appendix/release-notes/7.4.0.adoc

@@ -0,0 +1,6 @@
+[[eshadoop-7.4.0]]
+== Elasticsearch for Apache Hadoop version 7.4.0
+October 1, 2019
+
+ES-Hadoop 7.4.0 is a version compatibility release,
+tested specifically against Elasticsearch 7.4.0.

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-7.4.0>>
 * <<eshadoop-7.3.2>>
 * <<eshadoop-7.3.1>>
 * <<eshadoop-7.3.0>>
@@ -71,6 +72,7 @@ http://github.com/elastic/elasticsearch-hadoop/issues/XXX[#XXX]
 
 ////////////////////////
 
+include::release-notes/7.4.0.adoc[]
 include::release-notes/7.3.2.adoc[]
 include::release-notes/7.3.1.adoc[]
 include::release-notes/7.3.0.adoc[]