[Docs] Add instructions for building GenAI as OpenVINO extra module

src/docs/BUILD.md

@@ -37,6 +37,27 @@ The preferred approach is to build both OpenVINO and OpenVINO GenAI from sources
 
 ## Build Instructions
 
+### Build OpenVINO GenAI as OpenVINO Extra Module
+
+OpenVINO GenAI can be built as an extra module during the OpenVINO build process. This method simplifies the build process by integrating OpenVINO GenAI directly into the OpenVINO build.
+
+1. Clone OpenVINO and OpenVINO GenAI repositories:
+    ```sh
+    git clone --recursive https://github.com/openvinotoolkit/openvino.git
+    git clone --recursive https://github.com/openvinotoolkit/openvino_genai.git
+    ```
+2. Configure CMake with OpenVINO extra modules:
+    ```sh
+    cmake -DOPENVINO_EXTRA_MODULES=./openvino_genai -DCPACK_ARCHIVE_COMPONENT_INSTALL=OFF -S ./openvino -B ./build
+    ```
+3. Build OpenVINO archive with GenAI:
+    ```sh
+    cmake --build ./build --target package -j
+    ```
+
+After the build process completes, you should find the packaged OpenVINO with GenAI in the `build` directory.
+Follow the OpenVINO [build instructions](https://github.com/openvinotoolkit/openvino/wiki#how-to-build) and [install instructions](https://github.com/openvinotoolkit/openvino/blob/master/docs/dev/installing.md) for additional information.
+
 ### Build OpenVINO, OpenVINO Tokenizers, and OpenVINO GenAI From Source
 
 1. Build and install OpenVINO from sources following the [instructions](https://github.com/openvinotoolkit/openvino/wiki#how-to-build).