[DOCS] Adds placeholder for release notes (#1415)

Author: lcawl

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

@@ -0,0 +1,7 @@
+[[eshadoop-7.5.2]]
+== Elasticsearch for Apache Hadoop version 7.5.2
+
+coming::[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.adoc

@@ -8,6 +8,8 @@ This section summarizes the changes in each release.
 [float]
 [[release-notes-7]]
 ===== 7.x
+
+* <<eshadoop-7.5.2>>
 * <<eshadoop-7.5.1>>
 * <<eshadoop-7.5.0>>
 * <<eshadoop-7.4.2>>
@@ -76,6 +78,7 @@ http://github.com/elastic/elasticsearch-hadoop/issues/XXX[#XXX]
 
 ////////////////////////
 
+include::release-notes/7.5.2.adoc[]
 include::release-notes/7.5.1.adoc[]
 include::release-notes/7.5.0.adoc[]
 include::release-notes/7.4.2.adoc[]