[DOCS] Add 7.6.0 release notes (#1424)

Author: jrodewig

docs/src/reference/asciidoc/appendix/release-notes/7.5.2.adoc

@@ -0,0 +1,5 @@
+[[eshadoop-7.5.2]]
+== Elasticsearch for Apache Hadoop version 7.5.2
+
+ES-Hadoop 7.5.2 is a version compatibility release, tested specifically against 
+Elasticsearch 7.5.2.

docs/src/reference/asciidoc/appendix/release-notes/7.6.0.adoc

@@ -0,0 +1,5 @@
+[[eshadoop-7.6.0]]
+== Elasticsearch for Apache Hadoop version 7.6.0
+
+ES-Hadoop 7.6.0 is a version compatibility release, tested specifically against 
+Elasticsearch 7.6.0.

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

@@ -8,6 +8,9 @@ This section summarizes the changes in each release.
 [float]
 [[release-notes-7]]
 ===== 7.x
+
+* <<eshadoop-7.6.0>>
+* <<eshadoop-7.5.2>>
 * <<eshadoop-7.5.1>>
 * <<eshadoop-7.5.0>>
 * <<eshadoop-7.4.0>>
@@ -74,6 +77,8 @@ http://github.com/elastic/elasticsearch-hadoop/issues/XXX[#XXX]
 
 ////////////////////////
 
+include::release-notes/7.6.0.adoc[]
+include::release-notes/7.5.2.adoc[]
 include::release-notes/7.5.1.adoc[]
 include::release-notes/7.5.0.adoc[]
 include::release-notes/7.4.0.adoc[]