Add reference to release notes.

Author: schauder

src/main/asciidoc/upgrade.adoc

@@ -0,0 +1,8 @@
+[[new-features]]
+[[upgrading]]
+= Upgrading Spring Data
+
+Instructions for how to upgrade from earlier versions of Spring Data are provided on the project https://github.com/spring-projects/spring-data-commons/wiki[wiki].
+Follow the links in the https://github.com/spring-projects/spring-data-commons/wiki#release-notes[release notes section] to find the version that you want to upgrade to.
+
+Upgrading instructions are always the first item in the release notes. If you are more than one release behind, please make sure that you also review the release notes of the versions that you jumped.