Add documentation for DependsOn

docs/examples/junit4/generic/src/test/java/generic/DependsOnTest.java

@@ -0,0 +1,25 @@
+package generic;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.testcontainers.containers.GenericContainer;
+
+import static org.junit.Assert.assertTrue;
+
+public class DependsOnTest {
+
+    @Rule
+    // dependsOn {
+    public GenericContainer<?> redis = new GenericContainer<>("redis:3.0.2").withExposedPorts(6379);
+
+    @Rule
+    public GenericContainer<?> nginx = new GenericContainer<>("nginx:1.9.4").dependsOn(redis).withExposedPorts(80);
+
+    // }
+
+    @Test
+    public void testContainersAllStarted() {
+        assertTrue(redis.isRunning());
+        assertTrue(nginx.isRunning());
+    }
+}

docs/features/startup_and_waits.md

@@ -123,3 +123,11 @@ If none of these options meet your requirements, you can create your own subclas
 /startupcheck/StartupCheckStrategy.html) with an appropriate startup check mechanism in `waitUntilStartupSuccessful()`.
 Or you can leave it as is and just implement the `checkStartupState(DockerClient dockerClient, String containerId)` if you still want to check state
  periodically.
+
+## Depending on another container
+
+Sometimes, a container relies on another container to be ready before it should start itself. An example of this might be a database that needs to be started before your application container can link to it. You can tell a container that it depends on another container by using the `dependsOn` method:
+
+<!--codeinclude--> 
+[Depending on another container](../examples/junit4/generic/src/test/java/generic/DependsOnTest.java) inside_block:dependsOn
+<!--/codeinclude-->