Add missing upstream remote command to quickstart guide (nginx#2003)

Problem: The Quickstart instructions were missing the command
to the upstream remote

Solution: I added the missing command to add the upstream remote

docs/developer/quickstart.md

@@ -31,10 +31,10 @@ Follow these steps to set up your development environment.
    ```
 
 2. [Fork the project repository](https://github.com/nginxinc/nginx-gateway-fabric/fork)
-3. Clone your repository, and install the project dependencies:
+3. Clone your repository with ssh, and install the project dependencies:
 
    ```shell
-   git clone https://github.com/<YOUR-USERNAME>/nginx-gateway-fabric.git
+   git clone git@github.com:<YOUR-USERNAME>/nginx-gateway-fabric.git
    cd nginx-gateway-fabric
    ```
 
@@ -50,6 +50,12 @@ Follow these steps to set up your development environment.
    make deps
    ```
 
+4. Finally, add the original project repository as the remote upstream:
+
+   ```shell
+   git remote add upstream [email protected]:nginxinc/nginx-gateway-fabric.git
+   ```
+
 
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