Add reference to release notes

Closes gh-9646

Author: snicoll

README.adoc

@@ -64,6 +64,8 @@ Having trouble with Spring Boot? We'd like to help!
 * Learn the Spring basics -- Spring Boot builds on many other Spring projects, check
   the http://spring.io[spring.io] web-site for a wealth of reference documentation. If
   you are just starting out with Spring, try one of the http://spring.io/guides[guides].
+* If you are upgrading, read the https://github.com/spring-projects/spring-boot/wiki[release notes]
+  for upgrade instructions and ``new and noteworthy'' features.
 * Ask a question - we monitor http://stackoverflow.com[stackoverflow.com] for questions
   tagged with http://stackoverflow.com/tags/spring-boot[`spring-boot`].
 * Report bugs with Spring Boot at https://github.com/spring-projects/spring-boot/issues[github.com/spring-projects/spring-boot/issues].