docs: add a common developer.md file (#167)

Author: twishabansal

DEVELOPER.md

@@ -0,0 +1,37 @@
+# Development
+
+Below are the details to set up a development environment and run tests.
+
+## Install
+1. Clone the repository:
+    ```bash
+    git clone https://github.com/googleapis/mcp-toolbox-sdk-python
+    ```
+1. Navigate to the package directory:
+    ```bash
+    cd mcp-toolbox-sdk-python/packages/<PACKAGE_NAME>
+    ```
+1. Install the package in editable mode, so changes are reflected without
+   reinstall:
+    ```bash
+    pip install -e .
+    ```
+1. Make code changes and contribute to the SDK's development.
+> [!TIP]
+> Using `-e` option allows you to make changes to the SDK code and have
+> those changes reflected immediately without reinstalling the package.
+
+## Test
+1. Navigate to the package directory if needed:
+    ```bash
+    cd mcp-toolbox-sdk-python/packages/<PACKAGE_NAME>
+    ```
+1. Install the SDK and test dependencies:
+    ```bash
+    pip install -e .[test]
+    ```
+1. Run tests and/or contribute to the SDK's development.
+
+    ```bash
+    pytest
+    ```