Add breaking changes documentation for references PR (elastic#29963)

* Initial draft of breaking changes

* Add object structure change

* Make id attribute match

* Fix attribute name

* Remove relationships API notes

docs/migration/migrate_7_0.asciidoc

@@ -206,3 +206,43 @@ browsers like Internet Explorer 11 do not support CSP, we show them a warning me
 *Impact:* Nothing needs to be done necessarily, but if you don't need to support legacy browsers like IE11, we recommend
 that you set `csp.strict: true` in your kibana.yml to block access to those browsers entirely. If your organization requires
 users to use IE11, you might like to disable the warning entirely with `csp.warnLegacyBrowsers: false` in your kibana.yml.
+
+[float]
+=== Saved object structure change
+*Details:* Saved objects no longer contain ids of other saved objects under `attributes`. They have been moved under `references` with a named reference in `attributes`.
+
+Example structure before:
+```
+{
+  ...
+  "attributes": {
+    ...
+    "panelsJSON": [{
+      ...
+      "type": "visualization",
+      "id": "dbf71bb0-ffad-11e8-acfd-d359b3d9069"
+    }]
+  }
+}
+```
+
+Example structure after:
+```
+{
+  ...
+  "attributes": {
+    ...
+    "panelsJSON": [{
+      ...
+      "panelRefName": "panel_0"
+    }]
+  },
+  "references": [{
+    "name": "panel_0",
+    "type": "visualization",
+    "id": "dbf71bb0-ffad-11e8-acfd-d359b3d9069"
+  }]
+}
+```
+
+*Impact:* Users who query directly for ids will have to use the new `hasReference` in the find API. Users who save ids within attributes will have to move them into the `references` attribute and have a named reference to it.