Added note about skipping v5.1.0

Author: clintongormley

docs/reference/release-notes.asciidoc

@@ -5,6 +5,7 @@
 --
 This section summarizes the changes in each release.
 
+* <<release-notes-5.1.0>>
 * <<release-notes-5.0.2>>
 * <<release-notes-5.0.1>>
 * <<release-notes-5.0.0>>
@@ -19,6 +20,7 @@ This section summarizes the changes in each release.
 * <<release-notes-5.0.0-alpha1-2x>>
 
 --
+include::release-notes/5.1.0.asciidoc[]
 include::release-notes/5.0.2.asciidoc[]
 include::release-notes/5.0.1.asciidoc[]
 include::release-notes/5.0.0-all.asciidoc[]

docs/reference/release-notes/5.1.0.asciidoc

@@ -0,0 +1,7 @@
+[[release-notes-5.1.0]]
+== 5.1.0 Release Notes
+
+Version 5.1.0 doesn't exist because, for a short period of time, the Elastic
+Yum and Apt repositories included unreleased binaries labeled 5.1.0. To avoid
+confusion and upgrade issues for the people that have installed these without
+realizing, we decided to skip the 5.1.0 version and release 5.1.1 instead.