Add JUnit5/Jupiter usage instructions to docs (#954)

Author: kiview

docs/usage.md

@@ -151,11 +151,48 @@ public class SimpleMySQLTest {
 }
 ```
 
-### JUnit 5
+### Jupiter / JUnit 5
+
+#### Extension
+
+Jupiter integration is provided by means of the `@Testcontainers` annotation.
+  
+The extension finds all fields that are annotated with `@Container` and calls their container lifecycle 
+methods (methods on the `Startable` interface). Containers declared as static fields will be shared between test 
+methods. They will be started only once before any test method is executed and stopped after the last test method has 
+executed. Containers declared as instance fields will be started and stopped for every test method.
+  
+**Note:** This extension has only be tested with sequential test execution. Using it with parallel test execution is 
+unsupported and may have unintended side effects.
+  
+*Example:*
+```java
+@Testcontainers
+class MyTestcontainersTests {
+   
+     // will be shared between test methods
+    @Container
+    private static final MySQLContainer MY_SQL_CONTAINER = new MySQLContainer();
+    
+     // will be started before and stopped after each test method
+    @Container
+    private PostgreSQLContainer postgresqlContainer = new PostgreSQLContainer()
+            .withDatabaseName("foo")
+            .withUsername("foo")
+            .withPassword("secret");
+    @Test
+    void test() {
+        assertTrue(MY_SQL_CONTAINER.isRunning());
+        assertTrue(postgresqlContainer.isRunning());
+    }
+}
+```
+
+#### Vanilla Integration
 
-**JUnit5**: As of now, you need to manually start the container in a `@BeforeAll`/`@BeforeEach` annotated method in your tests. Tear down will be done automatically on JVM exit, but you can of course also use an `@AfterAll`/`@AfterEach` annotated method to manually call the `close()` method on your container.
+As an alternative, you can manually start the container in a `@BeforeAll`/`@BeforeEach` annotated method in your tests. Tear down will be done automatically on JVM exit, but you can of course also use an `@AfterAll`/`@AfterEach` annotated method to manually call the `close()` method on your container.
 
-Example of starting a container in a `@BeforeEach` annotated method:
+*Example of starting a container in a `@BeforeEach` annotated method:*
 
 ```java
 class SimpleMySQLTest {

modules/junit-jupiter/src/main/java/org/testcontainers/junit/jupiter/Testcontainers.java

@@ -11,8 +11,7 @@
  * {@code @Testcontainers} is a JUnit Jupiter extension to activate automatic
  * startup and stop of containers used in a test case.
  *
- * <p>The test containers extension finds all fields of type
- * {@link org.testcontainers.lifecycle.Startable} that are annotated with
+ * <p>The test containers extension finds all fields that are annotated with
  * {@link Container} and calls their container lifecycle methods. Containers
  * declared as static fields will be shared between test methods. They will be
  * started only once before any test method is executed and stopped after the