Fix cache sample README

Closes gh-5807

spring-boot-samples/spring-boot-sample-cache/README.adoc

@@ -43,6 +43,7 @@ If you want to configure your cache infrastructure via the standard, you need a
 implementation and the JSR-107 api. You first need to add `javax.cache:cache-api` to your
 project. Then you could try the following:
 
+* `EhCache 3`: add `org.ehcache:ehcache`
 * `Hazelcast`: add `com.hazelcast:hazelcast`
 * `Infinispan`: add `org.infinispan:infinispan-jcache`
 
@@ -59,14 +60,9 @@ of the library that you want to use.
 === EhCache 2.x
 Simply add the `net.sf.ehcache:ehcache` dependency to the project. Since there is a
 default `ehcache.xml` configuration file at the root of the classpath, it is automatically
-used to configure the underlying `CacheManager`.
-
-
-
-=== EhCache 3.x
-Simply add the `org.ehcache:ehcache` dependency to the project. Since there is a
-default `ehcache.xml` configuration file at the root of the classpath, it is automatically
-used to configure the underlying `CacheManager`.
+used to configure the underlying `CacheManager`. Note that EhCache 3 uses a different
+format and doesn't default to `ehcache.xml` anymore. Check
+http://www.ehcache.org/documentation/3.0/xml.html[the documentation] for more details.