Added Java 7 information

Author: tsundberg

examples/java-wicket/README.md

@@ -5,7 +5,7 @@ Selenium WebDriver and Cucumber-JVM using Java.
 
 ### Maven
 
-Simply run:
+Run:
 
 ```
 mvn install
@@ -14,3 +14,10 @@ mvn install
 This runs Cucumber features using the JUnit runner. The `@RunWith(Cucumber.class)` annotation on the `RunCukesIT` junit class
 kicks off Cucumber.
 
+### Java 7
+
+The cargo-maven2-plugin 1.3.0 is built using Java 7. This means that this example will fail if used with older Java versions. The error will be similar to:
+
+```
+[ERROR] Failed to execute goal org.codehaus.cargo:cargo-maven2-plugin:1.3.0:start (start-servlet-engine) on project java-wicket-test: Execution start-servlet-engine of goal org.codehaus.cargo:cargo-maven2-plugin:1.3.0:start failed: An API incompatibility was encountered while executing org.codehaus.cargo:cargo-maven2-plugin:1.3.0:start: java.lang.UnsupportedClassVersionError: org/eclipse/jetty/server/Server : Unsupported major.minor version 51.0
+```