doc: Improve the developer.md file to segregate install and test steps. (#99)

Author: anubhav756

DEVELOPER.md

@@ -1,11 +1,26 @@
-## Development
+# Development
 
-Below are the details to set up a development environment:
+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/genai-toolbox.git
     ```
+1. Navigate to the SDK directory:
+    ```bash
+    cd genai-toolbox/sdks/langchain
+    ```
+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 SDK directory:
     ```bash
     cd genai-toolbox/sdks/langchain
@@ -15,6 +30,7 @@ Below are the details to set up a development environment:
     pip install -e .[test]
     ```
 1. Run tests and/or contribute to the SDK's development.
+
     ```bash
     pytest
     ```