README.md: add usage example

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>

README.md

@@ -1,2 +1,35 @@
 # wiser-assertions
 Assertions for Wiser SMTP test server from subethamail
+
+## Usage
+
+    @Before
+    public void setUp() throws IOException {
+        wiser = new Wiser(PORT);
+        wiser.start();
+    }
+
+    @After
+    public void tearDown() {
+        wiser.stop();
+    }
+
+    @Test
+    public void testMail() {
+        //given
+        ...
+
+        //when
+        ...
+
+        //then
+        WiserAssertions.assertReceivedMessage(wiser)
+                .from(sender)
+                .to(recipient_alpha)
+                .to(recipient_beta)
+                .withSubjectContains(subject_prefix)
+                .withSubjectContains(subject_suffix)
+                .withContentContains(message_element_1)
+                .withContentContains(message_element_2)
+                .withContentContains(message_element_3);
+    }