docs: add root-level development.md guide for scoped builds

DEVELOPMENT.md

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+# Monorepo Scoped Development Guide
+
+This document outlines development workflows and build strategies when working with specific client libraries or modules in the `google-cloud-java` monorepo.
+
+---
+
+## 1. Building a Specific Service (Fast Reactor Builds)
+
+Instead of running a full build of the entire monorepo (which contains over 250+ modules and can take up to 30 minutes), you can target a single client library and let Maven automatically resolve and build all of its upstream dependencies using the `-pl` (project list) and `-am` (also make) flags.
+
+Run the following command from the **root directory** of the repository:
+
+```bash
+mvn install -pl java-spanner/google-cloud-spanner -am -P quick-build -DskipTests
+```
+
+### Flag Explanations:
+* **`-pl <module-path>`**: Targets only the specified project module.
+* **`-am` (Also Make)**: Tells Maven to analyze the dependency tree and automatically compile any projects in the reactor that the target library depends on (e.g., `gax-java`, `google-auth-library-java`, `google-cloud-core`).
+* **`-P quick-build`**: Bypasses unnecessary verification checks (like Checkstyle, Enforcer, Animal Sniffer) to significantly speed up local iteration.
+* **`-DskipTests`**: Skips running unit and integration tests during compilation.
+
+### Edge Case: Imported BOMs in `<dependencyManagement>`
+
+> [!WARNING]
+> **Maven's `-am` (Also Make) reactor analysis does not automatically build imported BOMs.**
+
+#### The Problem:
+If a module (or one of its parent dependencies/dependency BOMs) imports another BOM from within the same monorepo via `<scope>import</scope>` in a `<dependencyManagement>` block, Maven **does not** recognize it as a concrete build dependency.
+* *Example:* `google-cloud-bigtable-deps-bom` imports `google-cloud-monitoring-bom` via `<dependencyManagement>`.
+* Running `mvn install -pl java-bigtable/google-cloud-bigtable -am` (or targeting the deps-bom directly) will **succeed without actually building the local version of `google-cloud-monitoring-bom`**.
+
+#### The Solution (Targeted Build):
+To resolve this without doing a full 30-minute monorepo build, you can explicitly list the imported BOMs in the `-pl` parameter:
+
+```bash
+mvn install -pl java-monitoring/google-cloud-monitoring-bom,java-bigtable/google-cloud-bigtable-deps-bom,java-bigtable/google-cloud-bigtable -am -P quick-build -DskipTests
+```
+
+---
+
+## 2. Building All Submodules under a Service Directory
+
+Many services in this repository (e.g., `java-spanner`, `java-bigtable`) are structured as aggregator parents with multiple submodules (e.g., separate directories for client code, gRPC/Protobuf stubs, executors, and BOMs).
+
+Once your upstream monorepo dependencies are installed in your local ~/.m2/repository cache, you can run standard Maven commands for a specific service from the repository root using the -pl flag:
+
+```bash
+cd java-spanner
+mvn compile
+```
+
+Your IDE such as Intellij should also be able to import all the upstream dependencies at this moment. You can perform the same operations as you would in a normal project such as running unit tests, debugging and so on.