Protobuf Java and Kotlin code generation. (#64)

Co-authored-by: Stian Brandt <stian.brandt@it-smurfan.no>

api/java/README.md

@@ -0,0 +1,76 @@
+# chirpstack-api
+
+ChirpStack gRPC API message and service wrappers for Java.
+
+## Install
+
+An artifact is not yet public on Maven Central. You can build your own jar with
+
+```sh
+make jar
+```
+
+or install the artifact in your local Maven repository `$HOME/.m2` with
+
+```sh
+make install
+```
+
+## Usage
+
+All messages, services, constants, etc. are auto-generated from the ChirpStack protobuf definitions. The result is that
+this package structure matches that of the protobuf definitions.
+
+The protobuf definitions can be found here: https://github.com/chirpstack/chirpstack/tree/master/api/proto
+
+### Example
+
+#### Maven
+
+```xml
+<dependency>
+  <groupId>io.chirpstack</groupId>
+  <artifactId>chirpstack-api</artifactId>
+  <version>1.0-SNAPSHOT</version>
+</dependency>
+```
+
+#### Gradle
+
+```gradle
+dependencies {
+    implementation("io.chirpstack:chirpstack-api:1.0-SNAPSHOT")
+}
+```
+
+#### Example #1: List tenants
+
+```java
+import io.chirpstack.api.*;
+import io.grpc.ManagedChannel;
+import io.grpc.ManagedChannelBuilder;
+import io.grpc.Metadata;
+import io.grpc.stub.MetadataUtils;
+
+public class ChirpStack {
+    public static void main(String[] args) {
+        ManagedChannel channel = ManagedChannelBuilder
+            .forAddress("localhost", 8080)
+            .usePlaintext()
+            .build();
+        Metadata metadata = new Metadata();
+        Metadata.Key<String> key = Metadata.Key.of("authorization", Metadata.ASCII_STRING_MARSHALLER);
+        String token = "Bearer <INSERT_TOKEN_HERE>";
+        metadata.put(key, token);
+        TenantServiceGrpc.TenantServiceBlockingStub stub = TenantServiceGrpc
+            .newBlockingStub(channel)
+            .withInterceptors(MetadataUtils.newAttachHeadersInterceptor(metadata));
+        ListTenantsRequest request = ListTenantsRequest.newBuilder()
+            .setLimit(10)
+            .build();
+        ListTenantsResponse response = stub.list(request);
+        System.out.println("Number of tenants: " + response.getResultCount());
+    }
+}
+
+```