docs: Add a basic readme for kafka client (#320)

Author: dpcollins-google

pubsublite-kafka-shim/README.md

@@ -0,0 +1,90 @@
+# Instructions for Pub/Sub Lite Kafka usage.
+
+1. Add the following to your POM file to download the Pub/Sub Lite Kafka shim.
+```xml
+<dependency>
+  <groupId>org.apache.kafka</groupId>
+  <artifactId>kafka-clients</artifactId>
+  <version>2.6.0</version>
+</dependency>
+<dependency>
+  <groupId>com.google.cloud</groupId>
+  <artifactId>pubsublite-kafka-shim</artifactId>
+  <version>TODO: Make a release</version>
+</dependency>
+```
+
+1. Create a topic using `gcloud pubsub lite-topics create`
+1. Write some messages using:
+
+    ```java
+    import com.google.cloud.pubsublite.kafka.ProducerSettings;
+    import org.apache.kafka.clients.producer.*;
+    import com.google.cloud.pubsublite.*;
+    
+    ...
+    
+    private final static String ZONE = "us-central1-b";
+    private final static Long PROJECT_NUM = 123L;
+    
+    ...
+   
+   TopicPath topic = TopicPath.newBuilder()
+       .setLocation(CloudZone.parse(ZONE))
+       .setProject(ProjectNumber.of(PROJECT_NUM))
+       .setName(TopicName.of("my-topic")).build();
+    
+    ProducerSettings settings = ProducerSettings.newBuilder()
+       .setTopicPath(topic)
+       .build();
+   
+   try (Producer<byte[], byte[]> producer = settings.instantiate()) {
+       Future<RecordMetadata> sent = producer.send(new ProducerRecord(
+           topic.toString(),  // Required to be the same topic.
+           "key".getBytes(),
+           "value".getBytes()
+       ));
+       RecordMetadata meta = sent.get();
+   }
+    ```
+1. Create a subscription using `gcloud pubsub lite-subscriptions create`
+1. Read some messages using:
+
+    ```java
+    import com.google.cloud.pubsublite.kafka.ConsumerSettings;
+    import org.apache.kafka.clients.consumer.*;
+    import com.google.cloud.pubsublite.*;
+    import com.google.cloud.pubsublite.cloudpubsub.FlowControlSettings;
+    
+    ...
+    
+    private final static String ZONE = "us-central1-b";
+    private final static Long PROJECT_NUM = 123L;
+    
+    ...
+   
+    SubscriptionPath subscription = SubscriptionPath.newBuilder()
+        .setLocation(CloudZone.parse(ZONE))
+        .setProject(ProjectNumber.of(PROJECT_NUM))
+        .setName(SubscriptionName.of("my-sub"))
+        .build();
+   
+    ConsumerSettings settings = ConsumerSettings.newBuilder()
+        .setSubscriptionPath(subscription)
+        .setPerPartitionFlowControlSettings(FlowControlSettings.builder()
+            .setBytesOutstanding(10_000_000)  // 10 MB
+            .setMessagesOutstanding(Long.MAX_VALUE)
+            .build())
+        .setAutocommit(true);
+   
+    try (Consumer<byte[], byte[]> consumer = settings.instantiate()) {
+       while (true) {
+         ConsumerRecords<byte[], byte[]> records = consumer.poll(Long.MAX_VALUE);
+         for (ConsumerRecord<byte[], byte[]> record : records) {
+           System.out.println(record.offset() + “: ” + record.value());
+         }
+       }
+    } catch (WakeupException e) {
+       // ignored
+    }
+    ```