chore: refactoring README and DEVELOPMENT.md

README.md

@@ -1,6 +1,29 @@
 [![Coverage](https://sonarcloud.io/api/project_badges/measure?project=googleapis_gapic-generator-java&metric=coverage)](https://sonarcloud.io/summary/new_code?id=googleapis_gapic-generator-java)
-# API Client Generator for Java
 
-Generates a Java client library from protocol buffers. 
-Replaces the Java parts of the 
-[older monolithic generator](https://github.com/googleapis/gapic-generator).
+This repository consists of the following modules:
+
+- **[gapic-generator-java](./gapic-generator-java/README.md)**: the Protobuf compiler plugin to generate Java code.
+  See [gapic-generator-java/DEVELOPMENT.md] for setup.
+- **[gax-java](./gax-java/README.md)**: the runtime library required for Google Cloud client libraries,
+  including the ones generated by gapic-generator-java.
+- **[java-common-protos](./java-common-protos/README.md)** and **[api-common-java](./api-common-java/README.md)**: Protobuf-generated common
+  classes for Google services. (They are not generated by gapic-generator-java)
+- **[java-iam](./java-iam/README.md)**: Protobuf-generated classes for Google's
+  Identity and Access
+  Management (IAM). (They are not generated by gapic-generator-java)
+- **[showcase](./showcase/README.md)**: demonstration of the generated client
+  library for the fake "Showcase" API. 
+- **[gapic-generator-java-bom](./gapic-generator-java-bom)**: The Bill-of-Material for the libraries
+  produced from this repository. This is used by [google-cloud-java/java-shared-dependencies](
+  https://github.com/googleapis/google-cloud-java/blob/main/java-shared-dependencies/first-party-dependencies/pom.xml).
+
+## Run Tests for All Modules
+
+You need Java 11 or higher to run the build. The build produces Java 8-compatible
+bytecode.
+
+- Run all unit tests in all modules.
+
+    ```sh
+    mvn install
+    ```