Add Docker Compose plugin installation for Linux

Added instruction for Linux users to install Docker Compose plugin.

Author: simonLeary42

CONTRIBUTING.md

@@ -36,6 +36,7 @@ This repository will automatically check PRs for linting compliance.
 1. install PHP dependencies: `composer install`
 1. If you're on Windows, use [WSL](https://learn.microsoft.com/en-us/windows/wsl/)
 1. Download and install [docker desktop](https://www.docker.com/products/docker-desktop/)
+    1. Linux users only: Download and install docker compose plugin (`apt install docker-compose-plugin`)
 1. In `tools/docker-dev` Run the build script: `./build.sh`
 1. Run the environment: `./run.sh`. Press `CTRL+C` to exit
 1. Install [pre-commit](https://pre-commit.com/)