docs: Add simple explanation (#8)

README.md

@@ -1 +1,44 @@
 # k3a-embedded
+
+Embedded Kafka server for running simple integration tests. Embedded
+in the sense that the Kafka server is running in the same VM as the
+tests. No container runtime required.
+
+## Limitations
+
+* Not at all configurable. Starts a single-node server with no
+  security at all.
+* No Zookeeper. The server runs in KRaft mode.
+
+## What it does
+
+* Creates a temporary directory that will hold all runtime files of
+  Kafka. This directory is deleted when the server is stopped.
+* Finds a random available port that the broker will listen to.
+
+## Usage
+
+For JUnit tests, do something like this for starting, stopping, and
+getting the correct Kafka bootstrap servers (the latter will be
+`localhost` in combination with the random port):
+
+```java
+    private static K3aEmbedded kafka;
+
+    @BeforeClass
+    public static void beforeClass() {
+        kafka = new K3aEmbedded();
+        kafka.start();
+    }
+
+    @AfterClass
+    public static void afterClass() {
+        kafka.stop();
+    }
+
+    private Map<String, Object> getCommonConfig() {
+        final Map<String, Object> map = new HashMap<>();
+        map.put(CommonClientConfigs.BOOTSTRAP_SERVERS_CONFIG, kafka.getBootstrapServers());
+        return map;
+    }
+```