Add migration guide for 6.1

This commit adds the migration guide for 6.1.

Author: jasontedor

docs/reference/migration/index.asciidoc

@@ -20,3 +20,5 @@ As a general rule:
 See <<setup-upgrade>> for more info.
 --
 include::migrate_6_0.asciidoc[]
+
+include::migrate_6_1.asciidoc[]

docs/reference/migration/migrate_6_1.asciidoc

@@ -0,0 +1,15 @@
+[[breaking-changes-6.1]]
+== Breaking changes in 6.1
+
+[[breaking_61_packaging]]
+[float]
+=== Bootstrap checks now apply to link-local addresses
+
+Previously when Elasticsearch was bound to localhost, Elasticsearch would bind
+to any address on the loopback device. This included link-local addresses on the
+loopback device such as `fe80::1%lo0` on macOS. Starting in 6.1.2, Elasticsearch
+will now only bind to loopback addresses on the loopback device. Because of the
+previous behavior, Elasticsearch would skip the bootstrap checks if bound to a
+link-local address. Now that Elasticsearch no longer binds to such addresses
+when binding to localhost, the bootstrap checks now apply to link-local
+addresses.