chore: Add README for integrations folder

Co-authored-by: Kathy Wu <wukathy@google.com>
PiperOrigin-RevId: 875836816

Author: wukath

src/google/adk/integrations/README.md

@@ -0,0 +1,35 @@
+# ADK Integrations
+
+This directory houses modules that integrate ADK with external tools and
+services. The goal is to provide an organized and scalable way to extend ADK's
+capabilities.
+
+Integrations with external systems, such as the Agent Registry, BigQuery,
+ApiHub, etc., should be developed within sub-packages in this folder. This
+centralization makes it easier for developers to find, use, and contribute to
+various integrations.
+
+## What Belongs Here?
+
+*   Code that connects ADK to other services, APIs, or tools.
+*   Modules that depend on third-party libraries not included in the core ADK
+    dependencies.
+
+## Guidelines for Contributions
+
+1.  **Self-Contained Packages:** Each integration should reside in its own
+    sub-directory (e.g., `integrations/my_service/`).
+2.  **Internal Structure:** Integration sub-packages are free to manage their
+    own internal code structure and design patterns. They do not need to
+    strictly follow the core ADK framework's structure.
+3.  **Dependencies:** To keep the core ADK lightweight, dependencies required
+    for a specific integration must be optional. These should be defined as
+    "extras" in the `pyproject.toml`. Users will install them using commands
+    like `pip install "google-adk[my_service]"`. The extra name should match the
+    integration directory name.
+4.  **Lazy Importing:** Implement lazy importing within the integration code. If
+    a user tries to use an integration without installing the necessary extras,
+    catch the `ModuleNotFoundError` and raise a descriptive error message
+    guiding the user to the correct installation command.
+5.  **Documentation:** Ensure clear documentation is provided for each
+    integration, including setup, configuration, and usage examples.