[DOCS] Adds placeholder for alpha2 release notes

docs/reference/release-notes.asciidoc

@@ -6,8 +6,10 @@
 
 This section summarizes the changes in each release.
 
+* <<release-notes-7.0.0-alpha2>>
 * <<release-notes-7.0.0-alpha1>>
 
 --
 
+include::release-notes/7.0.0-alpha2.asciidoc[]
 include::release-notes/7.0.0-alpha1.asciidoc[]

docs/reference/release-notes/7.0.0-alpha1.asciidoc

@@ -1,5 +1,5 @@
 [[release-notes-7.0.0-alpha1]]
-== 7.0.0-alpha1 release notes
+== {es} version 7.0.0-alpha1
 
 The changes listed below have been released for the first time in Elasticsearch 7.0.0-alpha1.
 

docs/reference/release-notes/7.0.0-alpha2.asciidoc

@@ -0,0 +1,4 @@
+[[release-notes-7.0.0-alpha2]]
+== {es} version 7.0.0-alpha2
+
+coming[7.0.0-alpha2]