Add reference to release notes.

This adds a asciidoc snippet to be included in other modules documentation.
The snippet contains a reference to the release notes.

For simplicity the current version does not try to link directly to the proper version or module, but just references an overview which the user may use for further navigation.

Original pull request #2723

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.